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VELOCITY ANALYSIS AND DEPTH CONVERSION IN THE

OFFSHRE KABOUDIA PERMIT

General setting Depth Conversion Workflow

Data Input & QC

The Kaboudia permit lies in eastern offshore Tunisia and covers an area of about 3104 km².

It is bounded to the West by the Monastir-Mahdia shoreline and to the North east by the

Halk el Menzel Oil Field, the water depth is generally less than 200 meter.

The Kaboudia block is located in the Pelagian platform in the NW-SE “Mahdia-Isis”

paleohigh which separate the Gabes basin from the Hammamet basin. It lies in a highly

complex structural settings resulting from multiple extensional and compressional events

since late Triassic, these structural activities were associated with significant movements of

pre-existing faults and folds leading to the generation of many local unconformities,

structural inversion, erosion and lateral variation of facies of different stratigraphic series.

These complexities create several pitfalls in velocity model estimations due to lateral and

vertical heterogeneities and compounded by the lack of well control.

Previous depth conversion efforts utilized depth versus travel time curves derived from the

few available well velocities. We have adopted techniques such as geo-statistical modeling

in order to combine well and seismic stacking velocities therefore resulting in a more

accurate velocity model.This investigation represents a pilot study to develop depth

conversion methodologies which can be applied in other parts of the Pelagian platform. it is

the first step towards building a database of velocity models for the region.

The final depth maps and uncertainty analysis are a crucial input into reservoir modeling,

basin reconstruction, migration and maturation.

DATA INPUT OUTPUTS

WELL T-D CURVES

Stacking Velocities(+ DIX CONVERSION)

Well Section Velocity Analysis

Stacking Velocities CalibrationCalibration Coefficient Mapping

TWT SURFACES

Geostatistical AnalyisAverage Velocity mapping

Uncertainty Analysis

Depth ConversionHorizons

Faults

VELOCITY DATA ANALYSIS

3D Average Velocity Cube

Uncertainty and volumetrics

KEY METHODOLOGIES:

•Rigourous QC Of Input Data

•Geostatistical Modeling Of Stacking

Velocities Combined With Well Velocities.

•Depth Conversion Error Estimation

•Uncertainty Analysis

Constraints:

•Few Well Control.

•Smoothing Required to Achieve

Reasonnable Results.

•Seismic Stacking Velocities need to be

calibrated to well velocities.

METHODOLOGY:

•Velocity Data was gathered from three wells

and from 3D Seismic Survey

•Well Checkshots were Calibrated with

Sonic Logs to produce Final time-depth

curve for each well

•Well T-D curves are converted to interval

velocity and analysed for clear velocity

boundries

•Check each velocity dataset for errors

•Resampling of datasets to a common TWT

Interval

METHODOLOGY:

•Cross plot stacking velocity depths near a

well site with corresponding well depths

•Fit a linear polynomial to this cross-plot

and determine calibration coefficient from

the gradient of the polynomial.

•Grid calibration coefficients and Velocities

Using Geostatistic and trend analysis

•Multiply depths derived from Migration

velocities by calibration coefficient grid

Well T-D curves are converted to interval velocity and analysed

for clear velocity boundries

GeoStatisticalVelocity Analysis

Compilation of all migration velocities (blue dots) and

well velocities (coloured curves)

Well Depth

Mig

rati

on

Velo

cit

yD

ep

ths

•Two-way time–depth(TD)

functions from check shot

in MAH-2 well (Red curve)

compared to TD functions

from Migration velocity

adjacent to the well

location.The Curves in

black being the closest to

the well location

Inline 1249 (m)

Crossline 4015 (m)

Blue : Depth Grid from wells velocities

Red : Depth Grid using migration velocities

Yellow: Depth Grid Using General

Function

Green:Depth Grid From Wells And

Stacking (SGS)

•Cross-plot of depths

calculated using

Migration derived

velocities at several

locations adjacent to

MAH-2 well versus well

depths for MAH-2 well

After Calibration.

•A Variogram Map is Produced for each Layer and the major (Least variance) and minor directions

are determined. Sample variograms for the major and minor Directions are Generated and then

theoretical variogram is fitted to these.The final variogram is used as an input to the velocity map

Velocity map wells &Stacking

Velocity map (Stacking only)

2273 T-V pairs loaded in Stacking velocity dataset

Uncertainty and volumetrics

Z (

m)

OWT(s)

(a)-Difference between Well Depths and calibrated Stacking Depths as a

function of TWT in MAH-2 Well

(b)- difference between depth-converted seismic interpretation of the Serdj and

Abiod horizons and wells depths

(a) (b)

Cross-section in depth, the black bold curve is the base case and color

curves refer to different simulations

GRV distribution curve

Standard deviation

depth from Vstack

Standard deviation

depth from SGS

Vavg(wells) with

Vstack

The standard deviation maps gives an idea of how the error

increases when moving away from the wells

METHODOLOGY:

•The different depth maps can be obtained

by geostatistical conditional simulation to

generate precise samples that satisfy the

input statistics (mean, variance and

variogram),time-picking uncertainty has not

been considered.

•In order to calculate the gross volume of

the reservoir, the oil-water contact has to be

defined. The following statistics are easily

derived from the Gross Rock Volume (GRV)

distribution curve P90 (pessimistics

cenario),P50 (most realistic) and P10(

optimistic scenario).

Benefits:

•First stage of anew generation of depth

maps for Kaboudia Permit.

•Can be Combined with future models to

create a master velocity model database.

•Standardized methodology.