02yd0616

Vol. 45 No. 7 SCIENCE IN CHINA (Series D) July 2002

Reconstruction of the diagenesis of the fluvial-lacustrine-deltaic sandstones and its influence on the reservoir qualityevolution��Evidence from Jurassic and Triassic sandstones, Yanchang Oil Field, Ordos Basin

LUO Jinglan (��)1, S. Morad2, ZHANG Xiaoli (��)1,

YAN Shike (��)3, WU Fuli (��), LI Yuhong ( ��)

& XUE Junmin (��)3

1. Key Laboratory for Continental Dynamics, Ministry of Education of China; Department of Geology, Northwest Uni-

versity, Xi’an 710069, China;

2. Department of Earth Sciences, Uppsala University, S-752 36 Uppsala, Sweden;

3. Yanchang Petroleum Administration Bureau, Yongping 717208, China

Correspondence should be addressed to Luo Jinglan (email: [email protected])

Received December 14, 2001

Abstract The reservoir quality of Jurassic and Triassic fluvial and lacustrine-deltaic sandstones ofthe Yanchang Oil Field in the Ordos Basin is strongly influenced by the burial history and fa-cies-related diagenetic events. The fluvial sandstones have a higher average porosity (14.8%) anda higher permeability (12.7×10−3 µm2) than those of the deltaic sandstones (9.8% and 5.8 �10−3

µm2, respectively). The burial compaction, which resulted in 15% and 20% porosity loss for Juras-sic and Triassic sandstones, respectively, is the main factor causing the loss of porosity both forthe Jurassic and Triassic sandstones. Among the cements, carbonate is the main one that reducedthe reservoir quality of the sandstones. The organic acidic fluid derived from organic matter in thesource rocks, the inorganic fluid from rock-water reaction during the late diagenesis, and meteoricwaters during the epidiagenesis resulted in the formation of dissolution porosity, which is the mainreason for the enhancement of reservoir-quality.

Keywords: Yanchang Oil Field, Jurassic and Triassic fluvial lacustrine-deltaic sandstone, diagenesis, reservoir-quality

evolution.

The Ordos Basin is one of the important bases of energy resources in China. Commercial oil

and gas flows have been discovered in the Yan’an Formation of Jurassic and the Yanchang Forma-

tion of Triassic in the Yanchang Oil Field located in the Ordos Basin. The sandstone reservoir

quality in the Yanchang area displays a strong heterogeneity due to thin monolayers and great lat-

eral changes of the sandstones, their low porosity and permeability, the differentiation of

sedimentary facies, and the complicated burial history-related diagenesis as well. All of these

cause a great change of oil yield, which in turn, influences directly the hydrocarbon exploration

and development. Thus, unravelling the spatial and temporal distribution of diagenetic alterations

during burial and uplift is very important, because diagenesis plays a considerable role in the

preservation, enhancement and/or destruction of reservoir quality in sandstones[1�4].

No. 7 DIAGENETIC INFLUENCE ON RESERVOIR QUALITY EVOLUTION 617

Investigations on diagenetic alterations and their impact on the sandstone reservoir-quality

evolution of the Triassic and Jurassic fluvial and lacustrine-deltaic sandstone reservoirs from the

Ordos Basin have been made in this work, based upon the reconstruction of diagenesis during

burial and uplift processes. Emphasis is also placed on the elucidation of the influence of facies

and oil emplacement on the diagenetic

evolution of the sandstones.

The Yanchang Oil Field is located in

the northern Shaanxi slope, the western

Ordos Basin, which inclines gently to the

west with a dip angle of less than 1�.

Faults are not developed in the area, but

some noses in small scale.

The Yanchang Formation of the Up-

per Triassic is a suit of terrestrial fluivo-

lacustrine-deltaic sequence, consisting one

of the main hydrocarbon-producing inter-

vals in the area. The Upper Triassic

Yanchang Formation can be divided into

three facies from the bottom to the top (fig.

1): (i) lacustrine of Chang 7 Member

(T3Y31), composed of thick mudrocks and

oil shales, interbedded with thin layers of

very fine- to fine-grained sandstones, con-

stituting the important source rock in the

area; (ii) deltaic facies of Chang 6 (T3Y32)

and Chang 4+5 (T3Y33) Member; and (iii)

fluvial facies of Chang 2+3 (T3Y4) and

Chang 1 (T3Y5) Members. Among the del-

taic and fluvial facies, sandstones in

subaqueous distributary and distributary

channels in the deltaic, and channels in the

fluvial facies comprise the hydrocarbon

reservoirs. The individual sandstone layers

are a few centimetres to a few meters thick, but may form stacked bodies over 30 m thick. Typical

1) Li Wenhou, Chen Anning, Han Yongling, Studies on the depositional facies of the Yanchang Formation, southern Ordos

Basin, a research report in Chinese by the Yanchang Petroleum Administration Bureau and Department of Geology, Northwest

University, 1999.

Fig. 1. Typical stratigraphic section of the Lower-Middle Juras-sic and Upper Triassic fluvio-deltaic facies showing generallithostratigraphy and sedimentary structures (modified after LiWenhou, 1999)1). 1, Oil shale; 2, trough cross-bedding; 3, tubularcross-bedding; 4, parallel bedding; 5, ripple cross-bedding; 6,contorted bedding; 7, conglomerate; 8, gravel sandstone; 9, sand-stone; 10, silt; 11, muddy sandstone; 12, muddy silt; 13, siltymudstone; 14, mudstone. **, Low Jurassic (Fuxian).

618 SCIENCE IN CHINA (Series D) Vol. 45

sedimentary structures include bioturbation and trough and planar cross beddings. Sandstones are

mainly arkoses to lithic arkoses (Q45.8F42.9R11.2), with a small amount of feldspathic litharenite.

K-feldspar (av. 24%) dominates over plagioclase (av. 7%). The rock fragments are dominantly

sedimentary, granite, shallow metamorphic and volcanic rocks. Mica content, which is dominated

by biotite, is high (1%�15%, av. 4%).

The Yan’an Formation of the Middle Jurassic (Yan 10�Yan 4+5), deposited in a fluvial en-

vironment, constitutes another important hydrocarbon-bearing sequences in the study area. Chan-

nel-filling sandstones, which formed in a relatively intense hydrodynamic environment, typically

show medium- to large-scale cross-beddings, with a predominance of trough cross-bedding. Par-

allel bedding, ripple lamination and massive sandstones are also present. Sandstones are fine, me-

dium to coarse-grained lithic arkoses to feldspathic litharenite (Q61.9F30.8R7.6), with part of gravel

sandstones. Compared with the Triassic fluvial and deltaic sandstones, the Jurassic sandstones are

considerably rich in quartz and poor in feldspar (av. 20% and 5% for K-feldspar and plagioclase,

respectively) and biotite (0%�4%, av. 1%). The thickness of the sandstones is 8�20 m and can

be stacked up to 35�40 m.

Well logs of fifty four wells, penetrating the Yanchang (Triassic) and the Yanan (Jurassic)

formations, were prepared in order to determine the depositional facies and the general sequence

stratigraphic framework of the Triassic and Jurassic deposits. A total of 1549 routine analyses (a

total core length of 1056.72 m) of He-porosity and air-permeability were obtained for core plugs

from 41 wells. Core plugs covering a wide variation in facies and burial depths (847.54�2011.15

m) from twenty-one wells were sampled for the purpose of sedimentological, petrological and

geochemical studies. A total of 287 sandstone samples were selected for thin sections preparation

with vacuum-impregnation with blue epoxy resin. The modal composition and porosity were ob-

tained by counting 300 points in each thin section. Twenty-four samples were coated with a thin

layer of gold and examined with a JEOL JSM-T330 scanning electron microscope (SEM). Seven-

teen polished thin sections were coated with a thin layer of carbon for purpose of electron micro-

probe (EMP) analyses using a Cameca Camebax BX50 instrument equipped with three spec-

trometers and a black scattered electron detector (BSE). Carbon and oxygen isotope analyses of

carbonate cements were performed on seventeen representative samples. Six samples were se-

lected for strontium isotopes analysis. Cathodo-luminescence (CL) analysis was performed in 6

polished thin sections using a Technosyn cathodo-luminoscope. All analytical measurements were

done in the Department of Earth Sciences, Uppsala University, Sweden, except for the porosity

and permeability measurements.

1 Cements and their paragenetic sequence in sandstones

Cements in the sandstones include carbonate (av. 3.8%), clay minerals (av. 4.5%) and quartz

cements (av. 2.0%).


1.1 Carbonate cements

Carbonate cements in both the Triassic and Jurassic sandstones include calcite, ankerite and a

minor amount of siderite. Calcite is the main cement being more abundant in the Triassic (0�35%;

av. 7%) than in the Jurassic sandstones (0�7%, av. 2.2%). There are two types of calcite: type I

occurs as microsparry aggregates that fill intergranular pores and replace mud intraclasts and

pseudomatrix. It also fills intragranular spaces within expanded biotite grains. The grains enclosed

by this calcite are loosely packed and show no quartz overgrowths. Type II calcite occurs as

blocky to poikilotopic crystals 100�350 µm in size that fill the intergranular pores and gulfs, and

hence postdate type I calcite. Type II calcite replaces partially or completely the feldspar grains

and, to a less extent, quartz and rock fragments. Type II calcite engulfs quartz overgrowths, chlo-

rite, illite and ankerite. EMP analyses (table 1) revealed that type I calcite has slight higher Mn

and Mg than type II calcite. The average δ 13CPDB value (0.7�) of type I calcite is slightly lower

than that of type II calcite (+1.1�). The δ 18OPDB values and the 87Sr/86Sr values of type I and type

II calcites are much the same (see table 1).

Ankerite (0�12%, av. 1.4%) occurs as blocky, saddle (baroque) crystals, 100�350 µm in

size, which fill the intergranular pores, replace detrital feldspars and, to a less extent, pseudoma-

trix. It also occurs within expanded biotite grains. Ankerite engulfs, and thus post-dates, kaolin,

chlorite and illite grain coatings and quartz overgrowths. The contacts between calcite and

ankerite crystals display the evidence of slight replacement of ankerite by calcite. EMP analyses

of ankerite revealed a Ca-rich composition, a moderate to high Fe content and a low Mn content

(table 1). The average δ 13CPDB value is +0.8�, the average δ18OPDB value is −15�, and 87Sr/86Sr

= 0.712256. No intracrystalline crystal-chemical zonation has been detected in both the calcite and

ankerite.

Siderite (0�10%, av. 1%) occurs as small (3�8 µm) crystals arranged along traces of mica

cleavage planes, mainly in the fine-grained deltaic sandstones which are rich in mica and

pseudomatrix. Siderite occurs also as flattened rhomb (<3�15 µm) that are engulfed by, and

hence predate, kaolinite, chlorite, illite, ankerite and calcite. EMP analysis (table 1) revealed that

siderite is chemically zoned, with less magnesian cores (MgCO3 = 1.3%�3.7%; av. 2.3%) than

rims (MgCO3 = 17.7%). The core and rim contain low to moderate amounts of CaCO3 (1.3%�

9.3%; av. 4.9%).

1.2 Clay minerals

Diagenetic clay minerals in the sandstones include chlorite (av. 1.9%), illite (av. 1.2%), il-

lite/smectite-mixed layer (av. 0.8%), and kaolin (av. 0.6%). Chlorite occurs as extensive grain rims

comprised of pseudohexagonal crystals arranged perpendicular to the surfaces of framework

grains (fig. 2(a)). Chlorite fringes are more abundant in the deltaic, distributary channel and delta

front facies (0�15.7%; av. 3.2%) than in the fluvial sandstones (0�3%; av. 1%). Chlorite also


replaces biotite, volcanic fragments and, to a less extent, pseudomatrix and mud intraclasts. Chlo-

rite is covered or engulfed by, and hence predates, ankerite, calcite, illite and quartz .

Table 1 Major elements from microprobe analyses (average mole percent of 2�8 analytical points), oxygen and carbon

isotopic compositions (‰), and Sr-isotope ratio of the diagenetic carbonates

Well Depth/m AgeCarbonate

cementCaCO3 MgCO3 FeCO3 MnCO3 SrCO3

δ 18OSMOW

δ 18OPDB

δ 13CPDB

87Sr/86Sr

Z65-15 1548.6 T calcite I 94.3 1.4 2.8 1.3 0.1 10.8 −19.5 −1.7 0.713009

Z67-2 1529.4 T calcite I 8.7 −21.6 −0.1Z71-7 1610.5 T calcite I 17.1 −13.4 2.4

Z71-10 1613.1 T calcite I 8.8 −21.4 −0.2 0.712403

Z69-9 1674.4 T calcite I 97.1 0 1.2 1.4 0.5 0.713205

Z72-11 1701.5 T calcite I 96.2 0 1.4 1.8 0.7 10.5 −19.8 2.9

Z65-15 1548.6 T calcite I 97.4 0.3 1.3 0.6 0.53

Z71-26 1680.1 T calcite II 94.2 0.5 3.7 1 0.6

Z70-55 1657.4 T calcite II 95.4 0.6 1.7 1.7 0.3 8.4 −21.8 0.9

Z69-9 1674.4 T calcite II 93 1.9 3.3 1.6 0.3 8.5 −21.8 0.1

Z63-24 1733.6 T calcite II 96.2 0 1.4 0.9 1.5 0.712452

Z64-18 1332.2 T calcite II 97.4 0 0.7 1.8 0.1 8.8 −21.4 −3.1Z67-11 1607.3 T calcite II 95.6 0 1.9 1.5 1 11.1 −19.2 1.9 0.712837

Z71-29 1716 T calcite II 11.6 −18.72 3.7

Z71-26 1680.1 T calcite II 97.2 0.1 1.5 0.4 0.8 13.7 −16.7 2.2

Z64-1 1735.3 T calcite II 96.7 0.1 1.5 0.6 1.1 9.9 −20.4 1.3

Z70-48 1651.2 T calcite II 97.3 0 1.2 0.9 0.5 11.2 −19.2 1.3

Z70-55 1657.4 T calcite II 95.1 0.1 1.7 2.5 0.6

Z70-15 1564.1 T calcite II 96.3 0.1 1.4 1.5 0.6 9.2 −21.1 0.6

Z67-22 1721.7 T calcite II 99.9 0 0.1 0 0 12.7 −17.6 1.7

Z71-7 1610.5 T micro-ankerite 20.7 −10 0.9

Z71-10 1613.1 T micro-ankerite 17.2 −13.3 0.4

Z65-15 1548.6 T micro-ankerite 59.6 20.2 14.6 5.6 0

Z67-11 1607.3 T micro-ankerite 58.9 23.2 15.3 2.4 0.3 19.7 −10.9 2.1

Z63-1 1454.5 T sparry ankerite 56.6 24.3 14.5 4.7 0 13.6 −16.8 −2.4 0.712256

Z64-1 1735.3 T sparry ankerite 56.1 19 20.5 4.1 0.3

Z70-48 1651.2 T sparry ankerite 57.7 23.2 16.8 2.1 0.3

Z70-55 1657.4 T sparry ankerite 54.7 26.1 17.2 1.8 0.2

Z70-8 1021.5 J sparry ankerite 55.4 20.8 22.5 1.3 0

Z72-11 1701.5 T sparry ankerite 58 19.9 18 3.9 0.3

Z63-24 1733.6 T sparry ankerite 55.6 25.4 16.5 2.4 0.2 8.8 −21.5 2.6

Z71-32 1719.5 T sparry ankerite 56.3 23.3 18.1 2 0.2 12.9 −17.4 1.1

Z69-9 1674.4 T siderite core 8.4 2.1 87.4 2.2 0

Z70-8 1021.5 J siderite core 1.3 1.3 95.8 1.6 0

Z72-11 1701.5 J siderite core 1.6 3.7 92.4 2.2 0

Z70-8 1021.5 J siderite rim 5.2 17.7 75.5 1.6 0

Illite occurs in all facies as mat-like, fibrous or lath-like crystals that form rims around

framework grains and less commonly as pore filling cements. Illite is also formed by the replace-

ment of kaolinite, pseudomatrix and, less commonly, feldspars. Part of the grain-rimming illite

replaces a smooth layer of mechanically infiltrated, grain-coating smectite. Illite is engulfed by,

hence predates, quartz overgrowths, ankerite and calcite. SEM examinations revealed that kaolin


occurs as both kaolinite and dickite polymorphs. Kaolin is more abundant in the fluvial (0�10%,

av. 2.5%) than in the deltaic sandstones (0�5%, av. 0.4%). Kaolinite occurs as vermicular aggre-

gates and booklets of stacked thin platelets (10 �20 µm) partially replacing feldspars, micas, mud

intraclasts and pseudomatrix. In the fluvial Jurassic sandstones, kaolinite is etched and partially

replaced by well-crystallized, ordered kaolinite and/or euhedral blocky dickite (fig. 2(b)). Re-

placement of kaolinite by dickite is more extensive in the coarser-grained, permeable sandstones

than in fine-grained, less permeable sandstones. Kaolinite and dickite are embedded by, and hence

predate, quartz overgrowths, calcite and ankerite. Some of the kaolinite, particularly in the Triassic

sandstones, occurs as thin platelets that do not display evidence of dissolution, dickitization or

illitization. Such a kaolinite fills dissolution pores after calcite, selectively replaces plagioclase

grains and covers the quartz overgrowths, and thus this kind of kaolinite postdates quartz over-

growths and calcite cement.

Fig. 2. (a) BSE image showing that chlorite (chl) occurs as extensive grain rims comprised of pseudohexagonal crystals that arearranged perpendicularly to the surfaces of framework grains which have prevented quartz cementation, and thus the primarypores are preserved (q=detrital quartz grain) (Triassic deltaic sandstone, Well Z71, 1719.5 m). (b) SEM image showing inter-granular kaolin (kao) patches composed of thin etched platelets of kaolinite (kao) and thicker, euhedral crystals of ordered kao-linite/disordered dickite (di) (Jurassic fluvial sandstone, Well Z70, 1021.45 m). (c) SEM image showing quartz overgrowths (q)covering partially and engulfing chlorite (chl) rims (Triassic sandstone, Well Z64, 1333.66 m). (d) SEM image of albitized pla-gioclase showing euhedral, authigenic albite crystals (al) arranged parallel to each other. Remants of the detrital grain are indi-cated (arrows) (Triassic sandstone, Well Z71, 1541.0 m).

1.3 Quartz and albite cements

Quartz cement (trace to 7%; av. 2%) occurs as thin (< 50 µm thick) syntaxial overgrowths on


detrital quartz and, less commonly, as prismatic outgrowths in case the quartz grains are coated

with thick (< 250 µm) clay layers. Quartz cement covers and engulfs, thus postdates, authigenic

illite and chlorite coatings (fig. 2(c)), but is engulfed or partially replaced by, and thus predates,

ankerite and type II calcite cements. Quartz overgrowths are most abundant in coarse-grained,

fluvial-channel and in deltaic sandstones which are poor in carbonate cement and in

grain-rimming chlorite or illite. Quartz is also more abundant in the water-saturated than in the

oil-saturated sandstones. No correlation has been found between burial depths and abundance of

quartz cement. The fluid inclusions in quartz overgrowths are extremely rare and tiny (< 2 µm),

which thus precluded microthermometric measurements. CL examinations revealed the common

presence of fluorescing inclusions within quartz overgrowths.

Authigenic albite (trace to 10%, av. 3%) has replaced slightly to pervasively partially dis-

solved, detrital calcic plagioclase (An = 17%�32% mole percent) and, in rare cases, K-feldspar

grains (Ab = 12%�16% mole percent). The albitized grains are composed of numerous euhedral

crystals (2�10 µm) arranged parallelly to each other and to traces of twinning and cleavage

planes of the detrital feldspars (fig. 2(d)). EMP analysis further revealed that the albitized portions

have near-pure end-member composition (Ab > 99% mole percent). Some of the albitized plagio-

clases contain a thin layer of euhedral albite overgrowths. Albitized feldspars are most common in

permeable sandstones, and are cemented by, and hence predate, ankerite and type II calcite. There

is no systematic variation in the amounts of albitized plagioclase grains between the wa-

ter-saturated and oil-saturated sandstones, but albitized K-feldspar grains occur mainly in the wa-

ter-saturated sandstones.

2 Reservoir quality

The Jurassic and Triassic fluvial and deltaic sandstones contain both primary and secondary

porosities which were formed by the dissolution of framework grains (mainly feldspars and igne-

ous rock fragments) and calcite cement. Feldspar dissolution has resulted in oversized and moldic

pores, which often contain corroded remnants of feldspar. Dissolution of the rock fragment is of-

ten partial and has resulted in the formation of intragranular micropores (< 20 µm). The intra-

granular pores are often poorly connected with the intergranular pore system. The formation of

secondary intergranular pores due to cement dissolution is evidenced by the presence of scattered,

etched remnants of intergranular calcite cement and of empty pores adjacent to corroded quartz

overgrowths and grain-replacive calcite. The primary porosity represents the primary pores not

filled by the cements, and residual intergranular spaces partially occupied by the early cements or

by the compacted plastic fragments.

The overall core-plug and thin-section porosity values range from 2%�22.9% (av. 11.8%)

and 1%�18% (av. 7%), respectively. Permeability values are moderately correlated with

He-porosity, and vary between 0.01 and 1061×10−3 µm2 (av. 7.6×10−3 µm2). The fluvial sand-


stones have higher average porosity (14.8%) and permeability (12.7×10−3 µm2) than the deltaic

sandstones (9.8% and 5.8×10−3 µm2, respectively). Among the fluvial sandstones, the Jurassic flu-

vial sandstones have a reservoir-quality better (average porosity and permeability are 15.3% and

20.6×10−3 µm2, respectively) than the Triassic sandstones (average porosity and permeability are

14.3% and 7.6×10−3 µm2, respectively). The difference of about 5% between He- and thin-section

porosity values is mainly due to the presence of microporosity within clay mineral aggregates.

Average microporosity values are similar for both the deltaic and the fluvial sandstones. Overall,

deltaic sandstones have similar average

core-plug porosity values, but the delta-

plain sandstones have a much higher av-

erage permeability value (14.3×10−3 µm2;

(0.02�96.2)×10−3 µm2) than the delta

front (2.4×10−3 µm2; (0.01�700.3) ×10−3

µm2), pro-delta (1.1×10−3 µm2; (0.02�

45.1) ×10−3 µm2) and deep-water turbidi-

tic sandstones (2.3×10−3 µm2; (0.15�3.7)

× 10−3 µm2). Over the relatively narrow

depth range of the studied rocks (about

800 m), porosity and particularly perme-

ability values reveal a trend of slight de-

crease with increase in depth (r =

− 0.4428 and − 0.6048, respectively).

However, both porosity and permeability

values are strongly related to facies,

which, in turn, show a depth-related dis-

tribution. Porosity, and permeability in

particular, show a relatively weak posi-

tive correlation with increasing grain size

(r = 0.3448 and 0.4428, respectively) and

a considerably negative correlation with

total carbonate cement (r = − 0.6370 and

− 0.5270, respectively; fig. 3).

3 Burial-diagenetic evolution history

The Triassic and Jurassic fluvial and deltaic sandstones have undergone a variety of fairly

complex physical and chemical modifications. Detailed petrographic examination allowed us to

reconstruct the relative timing of the main diagenetic events and their influences on the reservoir

Fig. 3. Plots of porosity versus total carbonates (a), and perme-ability versus total carbonates (b) for the sandstones in the studyarea. Note that carbonate cementation was more intensive in thedeltaic sandstones than fluvial sandstones, and thus both porosityand permeability were deteriorated. 1, Meandering channel; 2,anastomosing channel; 3, delta plain; 4, delta front; 5, pro-delta; 6,lacustrine.


porosity and permeability. Many of these events are temporally overlapping each other and oc-

curred during early-, late- and epidiagenesis.

3.1 Classification of diagenetic phases and diagenetic evolution sequence

Classification of diagenetic phases in the study area is made based on Qiu Yinan et al.[5,6] and

Alaa et al.[4]. At the same time, paragenetic sequence of the main diagenetic minerals, especially

clay minerals, characteristics of the textures and organic matter in the rocks, origin of reservoir

spaces, the maximum temperatures of Rock-Eval pyrolysis, and the modeling burial-thermal his-

tory[7] are also considered. Thus, diagenesis of the Jurassic and Triassic sandstones includes three

phases: early-, late- and epidiagenesis. The boundaries between early-, late- and epidiagenesis are

not precise in terms of burial depths and temperatures. In this paper, the boundary between early-

and late-diagenesis is put at depths shallower than about 2 km and at temperature <70�[8], which

were pervaded during 225�138 Ma. Late diagenetic modifications are considered to occur during

the third rapid subsidence phase (138�23 Ma), and reached depths of 2.0�4.3 km and tempera-

tures of 70��140� (fig. 4), based on the present-day geothermal gradient. Epidiagenesis is

considered here to encompass alterations that occurred during the uplift to the depth shallower

than 2 km (23 Ma until today).

3.2 Evolution history of diagenesis

3.2.1 Early diagenesis. Early diagenesis was controlled by the depositional facies and detri-

tal composition. Early diagenesis mainly includes mechanical compaction, early diagenetic car-

bonate cementation, dissolution of the detrital fragments, and the mechanical infiltration of grain-

coating smectitic clay, and precipitation of kaolinite. Mechanical compaction reduced porosity and

permeability through increased grain packing and the bending and rupturing of mica and plastic

deformation of ductile rock fragments and mud intraclasts. Mechanical infiltration of smectitic

clay was mainly distributed in the point-bar and delta-plain facies. The early diagenetic carbonate

cementation (calcite I and siderite; >20 vol%) was concentrated in the fluvial-channel sandstones,

in siltstones and very fine-grained sandstones close to the organic-rich lacustrine and pro-deltaic

mudstones at parasequence boundaries of the early Highstand Systems Tract (HST). Interaction of

meteoric water with sandstones resulted in the dissolution of detrital fragments (mainly feldspars)

and the precipitation of kaolinite and smectite.

The carbon isotopic composition of diagenetic carbonate minerals can reflect the origin of

the carbon of which they are composed[9,10]. Type I micritic calcite cement is with narrow carbon

isotope values (δ 13CPDB of −1.7‰ to +2.9‰), which reflects an equilibrium relationship between

the carbon from the meteoric waters and the surface waters[11,12] . The 87Sr/86Sr value of the type I

calcite (av. 0.7126752) is higher than that of the sea during the late Triassic (87Sr/86Sr=0.7078)[13],


Fig. 4. The burial-thermal history and related diagenetic paragenesis of study area (R0 = 0.65% and R0 = 1.00% mean beginningof oil generation and peak oil generation, respectively).

which was the typical meteoric origin calcite. The relatively low oxygen isotope value (δ18OPDB of

−13.4‰ to −21.6‰) implies that type I micritic calcite precipitated either in highly 18O-depleted

meteoric water origin or at elevated temperature[4,14]. The paragenetic sequence of the authigenic

minerals and textures of the sandstones show that type I micritic calcite formed in the early

diagenetic phase. The proofs include: (i) sandstones are with high intergranular volume (IGV)

cemented by the type I calcite, (ii) framework grains of sandstones cemented by type I calcite

show floating contacts, and (iii) undeformed detrital biotite can be seen in the areas cemented by

the type I calcite. Hence, type I calcite is of meteoric precipitation origin. The extent and fre-

quency of carbonate cementation at parasequence boundaries (e.g. tendency to form continuous

cemented horizons that act as vertical barriers) increased towards the Maximum Flooding Surface

(MFS) due to the increasing in organic matter content and smectization and chloritization of the


biotite and volcanic fragments, and hence carbonate alkalinity of pore-waters. The relatively rich87Sr in type I calcite may be associated with dissolution of K-feldspar during the early diagenesis.

Small amounts of microcrystalline and low-Mg siderite precipitated within detrital biotite in the

deltaic facies. The biotite grains were expanded and pervasively filled adjacent pore spaces due to

their alteration into smectitic clays. Source for the carbonate cement may be derived from the

weathering products of Ca- and Mg-bearing minerals (e.g. plagioclase and biotite), and the disso-

lution of detrital feldspars during early diagenesis.

3.2.2 Late diagenesis. In addition to the increase in temperature and pressure, the spatial and

temporal distributions of the late diagenetic alterations, and hence of porosity-permeability evolu-

tion of the sandstones, were influenced by the early diagenetic modifications and oil emplacement.

Late diagenetic alterations include chemical compaction, the albitization of plagioclase, dickitiza-

tion and illitization of kaolinite, illitization and chloritization of smectite and the precipitation of

quartz, ankerite and type II calcite cements.

Kaolinite in the Jurassic fluvial sandstones buried to a maximum depth of about 2.5�3.5 km

(T ≈ 80 �110�; fig. 4) were subjected to progressive but partial transformation into thick book-

lets of well-ordered kaolinite and blocky crystals of dickite[15]. The transformation of kaolinite

into dickite occurs by dissolution and reprecipitation, as evidenced by the severely etched, thin

kaolinite platelets[16]. The more extensive replacement of kaolinite by well-ordered kaolinite and

dickite in the coarse-grained, permeable sandstones than in less permeable fine-grained sandstones

suggest that the reaction occurs in an open system involving fluid flux. Small amounts of

well-crystallized kaolinite occur in the vicinity of albitized plagioclase grains because albitization

resulted in the release of Al3+ and Ca2+, which precipitated to form small amounts of kaolinite and

calcite, as follows:

Na0.78Ca0.19Al1.19Si2.83O8+0.25H2O+0.33H+=0.78NaAlSi3O8+0.21Al2Si2O5(OH)4+0.19Ca2++0.08SiO2

plagioclase albite kaolinite

The inherited and mechanically infiltrated clay coatings, which are expected to be of smec-

titic composition particularly in the Triassic sandstones, were transformed into chlorite. This

transformation occurred either directly, or less commonly through the formation of an intermedi-

ate, early diagenetic S/C phase. However, the transformation of smectite into pseudohexagonal

chlorite crystals that are arranged perpendicularly to grain surfaces occurred adjacent to open

pores, but not at intergranular contacts. Chloritization occurred in the deltaic sediments that are

rich in biotite and volcanic rock fragments, which acted as sources of Fe2+ and Mg2+. Additional

sources of Fe2+ and Mg2+ include fine crystalline, grain coating iron-oxide pigments which are

commonly attached to the infiltrated clays.

In the Triassic sandstones, the etched, thin platelets of early diagenetic kaolinite were

strongly illitized, but dickite booklets were not affected due to its better-ordered crystal lattice[16].

Illite was also formed by the replacement of smectitic grain coatings through an intermediate I/S


phase. A limited extent of albitization of detrital K-feldspar occurred simultaneously with chlori-

tization of biotite, and thus acted as internal sources of potassium used in the illitization reactions.

Extensive albitization of detrital K-feldspar is most pervasive at higher temperatures (�130�)[8]

than those required for plagioclase albitization.

Quartz precipitation occurred during a wide range of burial depths, but mainly subsequent to

the formation of illite and chlorite. The main source of silica is the localized intergranular pressure

dissolution of detrital quartz, which is common in the quartz-cemented sandstones. Other minor

sources include the illitization of smectite and kaolinite and the dissolution of detrital feldspar and

rock fragments. The residual primary pores after quartz cementation were occupied by ankerite

and calcite II. Ankerite is a typical late diagenetic cement in many other siliciclastic sequences

worldwide[17,18]. Fe2+ and Mg2+ needed for ankerite and chlorite cementation are derived internally

from dissolution of volcanic fragments, biotite and grain coating smectitic clays and iron-oxide

pigment, and externally by diffusive transfer from adjacent mudrocks[17]. Sources of Ca2+ include

rhizocretionary and early diagenetic calcite and, to a minor extent, calcic plagioclase. Minor

amounts of Mg-rich siderite precipitated as overgrowths on Mg-poor, early diagenetic siderite.

Mg-rich but Ca-poor siderite typically formed during late diagenesis[18].

Paragenetically, type II calcite postdates siderite and late diagenetic ankerite, and thus formed

during either deep-burial late diagenesis or uplift and epidiagenesis. The oxygen isotopic, and es-

pecially carbon isotopic compositions of the ankerite and calcite II, increase with burial depth (r =

0.4211 and 0.8400, respectively)�implying that carbonate cements were influenced by the bur-

ial-diagenesis[12]. The oxygen isotopic values of ankerite and calcite II are extremely low (av.

δ 18OPDB = −15‰ and −19.8‰, respectively), indicating that they were originated either from pre-

cipitation of highly 18O-depleted meteoric water or formed at elevated temperatures. Assuming a δ18OSMOW formation water composition as low as −6‰ and −4‰, which occurred in deep basinal

brines (�3 km) mixed with meteoric waters in other basins[19], and using the fractionation equa-

tion of Friedman and O’Neil[20], the temperature for calcite II and ankerite precipitation would be

around 80 �140� and 80 �145�, respectively. Such temperatures agree well with the maxi-

mum temperatures reached during late diagenesis (110 �145�), implying that calcite II and

ankerite mainly formed during the late deep burial diagenesis. The relatively high carbon isotopic

value of type II calcite may be related to a mixed source of carbons derived from meteoric waters

and thermal methanogenesis during the hydrocarbon emplacement.

The emplacement of oil has strongly retarded the precipitation of quartz and carbonate ce-

ments, which are much more abundant in the water-saturated reservoirs than in the oil-saturated

sandstones, because hydrocarbon is a mixture of poly-hydrocarbons, which have no ability to dis-

solve and precipitate inorganic salts of which authigenic minerals are composed. Thus, the accu-

mulation of organic matter in the pores effectively inhibited the diagenesis. Oil emplacement did

not influence the formation of grain-rimming, illite and chlorite. Conversely, the transformation of


the clay minerals during late diagenesis promoted the accumulation of hydrocarbons in the reser-

voirs. Studies show that transformation of clay minerals in the 1000�2800 m depths can play a

considerable role of catalytic effect for the gas generation of organic matter[21]. Among clay min-

erals, smectite is with the strongest catalytic effect which may enhance for 2�3 times the hydro-

carbon generation output, and at the same time, result in 50� decrease in temperature of

Rock-Eval pyrolysis. During the transformation of smectite, the replacement of aluminum for sil-

ica will take place, which leads to the acidic surface of smectite due to unequilibrium of charges.

3.2.3 Epidiagenesis. The uplifting of sedments to the depths shallower than 2 km (T<70�)

promoted the meteoric water invasion, leading to the dissolution of silicate grains such as feld-

spars (mainly plagioclase) and precipitation of kaolinite. The extent of plagioclase kaolinitization

varies widely even within the same well, being most extensive in the medium- to coarse-grained

fluvial sandstones. Compared with the early diagenetic kaolinite, which is most common in the

Jurassic sandstones, late-epidiagenetic kaolinite reveals no evidence of dickitization or illitization.

Meteoric-water incursion into the reservoirs is evidenced by the presence of formation-waters

with a brackish composition. The dilute, present-day formation waters have δ 18OSMOW as low as

−11.3‰, and δ D values between −80.4‰ and −71.5‰. These values fall close to the Global Me-

teoric-Water Line (GMWL), suggesting a dominantly meteoric-water component. The wide varia-

tions in formation-water composition are attributed to the extent of mixing between the basinal

brines and the downward flowing meteoric waters.

4 Burial-diagenetic modifications on reservoir quality

The fairly complex variety of physical and chemical diagenesis strongly influenced the Ju-

rassic and Triassic sandstone reservoir quality. These modifications were closely related to the

depositional facies, emplacement of oil, and a complex subsidence and uplift history of the basin.

4.1 Effect of compaction on reservoir quality

Mechanical compaction during the early diagenetic phase reduced porosity and permeability

through increased grain packing, displacement and rearrangement of the detrital fragments, plastic

deformation of the ductile mud intraclasts to pseudomatrix as well as bending and squeezing of

biotite between rigid grains.

The overall small amounts of early diagenetic cements rendered compaction to be more im-

portant than cementation in reducing porosity and permeability. Determination of the depositional

intergranular porosity is difficult, but assuming an original porosity value of 40%, the average

porosity loss due to compaction is about 15% (ranges from 4%�25%) in the Jurassic fluvial

sandstones and 20% (ranges from 3% to 32%) in the Triassic deltaic sandstones (fig. 5). This ac-

counts partly for the lower porosity and permeability in the Triassic deltaic sandstones (2%�20%;

av. 9.8%, and (0.01�700.3) ×10−3 µm2; av. 5.8×10−3 µm2, respectively) than in the Jurassic


fluvial sandstones (5%�22.9%; av. 15.3%,

and (0.05�1061)×10−3 µm2; av. 20.6×10−3

µm2, respectively). The slightly greater loss

of porosity and permeability due to com-

paction in the deltaic sondstones than the

fluvial sandstones is partly attributed to the

higher content of ductile grains such as

mica, argillaceous rock fragments and mud

intraclasts. In addition to grain rearrange-

ment, the most important diagenetic proc-

ess in deteriorating the reservoir properties

is the plastic deformation of such ductile

grains between the rigid feldspar and quartz

grains. Studies on burial modifications of

the intergranular porosity performed on a

large number of sandstones[22,23] revealed

that IGV values decrease rapidly by 28% mainly by grain rearrangement at a depth of 1500 m, and

then more slowly to 26% at the depth of 2400 m. Thus, mechanical compaction during the early

diagenesis and early stage of the late diagenesis (depth <2500 m) is attributed to the loss of poros-

ity of the sandstones in the study area. The higher feldspar, rock fragments and ductile fragments

(e.g. mica) contents in the deltaic Triassic sandstones than in the Jurassic sandstones probably

contributed to the tighter packing, especially the biotite which expanded and alterated into smec-

tite (the expansion rate of smectite is 98.2% under saturated water condition)[5] and/or precipita-

tion of microcrystalline carbonate along traces of its cleavage planes caused efficient choking of

all adjacent pore throats, and hence greater compaction loss of porosity and permeability. Fur-

thermore, the higher detrital quartz content in the Jurassic sandstones than in the Triassic sand-

stones resisted the effect of the mechanical compaction and thus resulted in the preservation part

of the primary porosity in the Jurassic sandstones.

The intergranular pressure dissolution and concomitant local precipitation of dissolved silica

as quartz overgrowths induced an additional loss of porosity and permeability. Pressure dissolu-

tion is most extensive in sandstones rich in illitic grain coatings and mica, which are believed to

have a catalytic effect on this process[24]. The expansion of abundant biotite grains, particularly in

the deltaic sandstones, induced the deterioration of reservoir quality by both alteration and expan-

sion and by the enhancement of intergranular pressure dissolution of quartz grains.

4.2 Effect of cementation on reservoir quality

The average original porosity losses due to cementation are similar in the fluvial and deltaic

sandstones (10%; in a range of 2%�26%; fig. 5). The precise role of cementation on reservoir

Fig. 5. Plot of intergranular volume (%) versus cement (%) forthe Jurassic-Triassic fluvio-deltaic sandstones with a deposi-tional intergranular volume of 40% (see ref. [14]). Note thestronger influence of compaction than cementation in the de-struction of porosity.


quality is fairly complex due to the presence of several types of cements with various textural

properties and occurrence habits. However, carbonates are the most important cements that caused

deterioration of both permeability and porosity (see fig. 3) and, to a less extent, quartz over-

growths. Early diagenetic carbonate cementation, particularly in the Triassic sequence, tends to

occur along parasequence boundaries in the deltaic facies of the early HST. They formed laterally

cemented layers, and thus potential reservoir barriers in field scale. Late diagenetic carbonates

occur as less pervasive cement than the early diagenetic type I calcite, but are more frequent, more

homogeneously distributed, and hence have a greater influence on the porosity-permeability loss

in the sandstones. Carbonate cements are more abundant, and hence have a greater control on

porosity and, particularly, permeability of the deltaic sandstones than of the fluvial sandstones. A

higher overall content of carbonate cement of the delta-front and pro-delta sandstones (av. 4.0%

and 4.2%, respectively) accounts partly for the considerably lower reservoir quality (av. perme-

ability is 2.4×10−3 µm2 and 1.1×10�3 µm2, respectively) compared with the delta-plain sandstones

(av. carbonate cement is 3.9% and av. permeability is 14.3×10−3 µm2, respectively). However, the

formation of a small amount of early diagenetic quartz cement and homogeneously distributed

early diagenetic carbonates in sandstones weakened part of the mechanical compactional effect,

and hence the preservation of part of the primary porosity in sandstones.

The clay minerals have variable influence on reservoir quality evolution. The grain-rimming

chlorite and, particularly, illite caused a substantial deterioration of reservoir quality by decreasing

the permeability through blocking the pore throats and by increasing the microporosity. Neverthe-

less, the presence of chlorite rims around quartz grains inhibited the precipitation of quartz over-

growths, and hence contributed to the preservation of primary intergranular porosity, yet often low

permeability, in the deltaic sandstones and less commonly in the fluvial sandstones. Sandstones

with more than 5% (volume percent) of chlorite have porosities greater than 10% and a relatively

low permeability range ((0.1�10)×10−3 µm2). In the absence of quartz overgrowths, the inter-

granular pores lined by chlorite were in some of the water-saturated sandstones filled fairly exten-

sively by late diagenetic ankerite. The illitization of intergranular kaolinite, and hence further de-

terioration of permeability, was inhibited due to the earlier transformation of kaolinite into dickite,

which is more resistant to illitization due to its better ordered crystal structure. The retardation of

kaolinite illitization was also due to the only small degree of albitization of K-feldspar grains,

which are the most important sources of potassium ions.

4.3 Effect of dissolution on reservoir quality

The coarse-grained Jurassic fluvial sandstones preserved a large part of their primary poros-

ity after modification by compaction and cementation, which provided the spatial bases for form-

ing the secondary dissolution. Thus, the dissolution of feldspar (mainly plagioclase) is more

common in the Jurassic fluvial sandstones than in the finer-grained Triassic deltaic sandstones.

The acidic pore-fluids, rich in organic and inorganic acids, were the main dynamic force and


fluid medium that resulted in the dissolution of fragments, and were also the carrier for part of

reaction production migration. Study on the burial-diagenetic history and paragenetic sequence of

diagenetic processes shows that migration and accumulation of oil in the area occurred simulta-

neously with the main diagenetic events in late diagenesis (see fig. 4). During this period, organic

and inorganic reactions in the reservoir-source rock system were very active. Furthermore, organic

matter in the source rocks had entered A2 period (organic matter mature period) and B period (or-

ganic matter highly mature period)[5,6] during the late diagenesis1). The transformation of organic

matter into hydrocarbons in the source rocks would release a large amount of CO2, which might

cause pore-fluids to be acidic[25,26]; with the burial depth and temperature increase, the kerogen in

mudstones will form some organic acids such as carboxylic acid in the temperature range of 80

�120�, due to the thermal splitting of the oxygen-bearing functional group, which can lead to

the increase of dissolubility of Al-silicates and carbonates and result in their dissolution[24]; more-

over, transformation of clay minerals may produce acidic fluids. For example, transformation of

kaolinite into illite can be associated with the formation of some amount of H+[27]:

3Al2Si2O5(OH)4 (kaolinite)+2K+�2KAl3Si3O10(OH)2 (illite) + 3H2O + 2H+

Sulphates’ reaction with hydrocarbons in 100 �140� (mature period for organic matter)

during deep burial late diagensis will produce H2S, which was dissolved into the water, resulting

in the formation of acidic water[27]. The formation water in the study area was acidic to weak

acidic with CaCl2-type water as its main water type and pH values mainly in the range of 5.5�6.8

(table 2), which could help to dissolve acidic-dissolving materials.

Table 2 Chemical composition of the formation water in the Yanchang areaa)

Region� Age� Depth/m� Cl− SO42− Ca2+ Mg2+ K++Na+ pH Sample� Water type�

Fengzhuang� �� 496 � 611 92.9 1 6.2 2.9 90.9 5.8 4 CaCl2

Zhidan� �� 1504 � 1773 97.9 1.7 15.8 2.9 81.2 5.5 6 CaCl2

Qinghuabian� �� 260 � 346 91.5 6.2 37.4 6.5 56.1 6.8 11 CaCl2

Zichang� �� 258 � 1169 97.2 6.2 77.7 6.5 15.8 6.5 2 CaCl2

Yujiaping� �� 354 � 489 66 15.2 30.3 1.6 68.2 8 9 CaCl2

a) The ions are accounted in weight percent.

The acidic pore-fluids caused by the above factors led to the dissolution of detrital feldspars

(mainly plagioclase) and the early diagenetic cements (mainly calcite), during which the precipita-

tion of kaolinite was accompanied:

CaCO3 (calcite)+ CO2 + H2O � 2HCO3− + Ca2+

MgCa(CO3)2 (ankerite)+ 2CO2 + 2H2O � 4HCO3− + Ca2+ + Mg2+

KAlSi3O8 (K-feldspar)+2H�+H2O�Al2Si2O5(OH)4 (kaolinite)+4SiO2 (quartz)+2K�

2Na0.6Ca0.4Al1.4Si2.6O8 (plagioclase)+1.4H2O+2.8H�

�1.4Al2Si2O5(OH)4 (kaolinite)

1) Luo Jinglan, Zhang Xiaoli, Chen Zhenjiang et al., Reservoir discription for the Jurassic and Triassic Formation, Zhidan

exploration area, Yanchang Oil Field (in Chinese), 1998, 73�85.


+1.2Na�+0.8Ca+2.4SiO2 (quartz)

The dissolution of the detrital grains (mainly feldspar) and carbonate cements, and the pre-

cipitation of kaolinite occurred during epidiagenesis under influx of meteoric waters. The late- and

epidiagenetic kaolinite filled the dissolution pores of intra-feldspar and intra-calcite cement, and

thus, kaolinitization was accompanied with dissolution of the calcite. Generally, kaolinitization of

the plagioclase is more intensive in the medium- to coarse-grained fluvial sandstones than in the

very fine- to fine-grained deltaic sandstones. Thus, selective late diagenetic dissolution of plagio-

clase contributed to the domination of detrital K-feldspar over plagioclase, and probably to the

overall lower average of plagioclase and K-feldspar contents in the more permeable and better

interconnected fluvial sandstones (5% and 20%, respectively) than in deltaic sandstones (7% and

24%, respectively).

5 Conclusions

(1) Higher content of detrital quartz and lower content of feldspars and ductile fragments in

the Jurassic fluvial sandstones resisted the effect of compaction, which resulted in the partial pres-

ervation of the primary porosity. The presence of somewhat greater amounts of feldspar and duc-

tile fragments, and micas in particular, in the Triassic deltaic sandstones, together with the

fine-grained texture of these sediments, resulted in poorer reservoir quality.

(2) The late diagenetic ankerite, calcite, and less commonly, quartz cementation caused a

considerable decrease in porosity and permeability in the water-saturated zone, but were strongly

inhibited in the oil-saturated zone. The amounts of late diagenetic grain-fringing chlorite and illite

were not greatly influenced by oil emplacement. Conversely, they may promote oil accumulation

to some extent.

(3) The grain-coating chlorite and illite resulted in a considerable decrease in permeability.

Nevertheless, chlorite fringes induced the preservation of a considerable proportion of the inter-

granular macroporosity through inhibiting the precipitation of quartz overgrowths.

(4) The late diagenesis and epidiagenesis are the main phases for the formation of secondary

dissolution pores, which result in the dissolution of detrital plagioclase and calcite cement, and

thus enhancement of porosity. The acidic pore-fluids rich in organic and inorganic acids, as well as

the meteoric waters, are the main dynamic force and fluid medium for the dissolution of fragments

in the sandstones. Plagioclase dissolution was associated with the formation of kaolinite, which

typically shows no evidence of dickitization or illitization.

Acknowledgements The constructive comments made by the reviewers and much help from Mr. Li Wenfan duringmodification of the paper are acknowledged with thanks. We thank the Academic Institute of Yanchang Petroleum Exploration forproviding part of the petrophysical data and formation-water analyses. Zhidan Petroleum Exploration and Development Head-quarters kindly provided access to the cores. Luo Jinglan thanks the China Scholarship Council (CSS) for supporting herone-year research work in Sweden. S. Morad thanks the Swedish Natural Sciences Research Council (NFR) for funding theresearch studies on diagenesis in Uppsala. This work was supported by the National Natural Science Faundation of China (GrantNo. 40192055).


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