3D SEISMIC SURVEY IN THE GAS-OIL FIELD OFFSHORE KATAKOLON (WESTERN GREECE) DEVELOPING LOW COST TECHNIQUES

Ziel

The 3D seismic survey project covered the area of the oil and gas field discovered in 1980 by Public Petroleum Corporation. The field name "West Katakolon" is located 3 miles offshore Cape Katakolon in Western Greece.
The producing horizon is the anticlinal Cretaceous carbonates unconformably covered by clastic Neogene sediments. A prominent feature SE from the structure is an evaporitic diaphir which cuts through the carbonates and the younger Neogene strata.
The results from the tests in the 3 wells that followed the discovery proved that the field is classified as marginal mainly due to the weater depth in the area which is about 230 m.
The need for increased confidence in the estimation of reserves led towards the planning of a "low cost" 3D seismic survey which is considered as the appropriate approach for financially marginal oil fields. Other alternative is the conduct of appraisal wells a far dearer solution than specially dedsigned 3-D surveys.
The need for increased confidence in the structural interpretation in the case of marginal fields led towards the planning of a 3-D seismic survey which could provide a better basis for reserves estimation.
The area of the 3-D coverage is rectangular in shape with dimensions 13 x 7.5 km. its long axis running NW-SE.
The two novel techniques successfully applied in the Acquisition and processing of the data dramatically reduced the cost and proved the 3-D survey applied in this project to be a cost effective technique for the development of marginal oil fields such as the W. Katakolon.
Following the interactive interpretation it appears that.
1. The evaportic diaphir is now interpreted over a more restricted zone allowing space for mapping the base of Neogene unconformity over a more extended area compared to previous results (2-D seismic).
This unconformity being the top of the producing horizon is a key target for oil exploration in Western Greece.
2. The configuration of the W.Katakolon structure is now different presenting two isolated highs west of the main structure. Also the peak of the main structure appreared at a shallower time contour, i.e. the structure is thicker.
3. An independent high SE of the evaporite is revealed extended over an area comparable in sizesize to the W. Katakolon structure.
4. Conversion of time to depths is considered to be much more accurate than previous attempts. The water bottom and two Neogene horizons A and B were mapped. Velocities from well log data of W. KA-2 were assigned to the layers in between the mapped horizons. The depth maps produced are free of the water bottom effect. This resulted in a slightly different configuration of the structure when compared with the time map. One isolated high instead of two is now observed west of the main structure.
More accurate volumetric calculations are now possible on the basis of the depth maps. The figures of interpretation supplied by G.S.I. show an increase of 15% approximately for the volume of the Reservoir rock. The new volumetric data along with the revealed individual highs are now being studied by our reservoir engineers and a more accurate reserve estimation is expected.
To meet the requirements of detailed structural delineation high resolution in the final product was necessary.
Close line spacing in the acquisition stage preserves higher frequencies in recording the seismic wave field. Generally the higher the frequencies recorded the higher the resolution achieved.
In the calculation of the desired line spacing enter data such as the desired frequency the velocity of the reflector and its maximum dips. A line interval of 37.5m. was considered necessary to assure the desired high frequency to be recorded. For such a close line spacing requirement, mainly due to steep dips present in West Katakolon the conventional technique of collecting one line of sub-surface per vessel pass would make the survey very expensive to be applied in such marginal field.
To meet the requirements for a low-cost technique the following innovation was designed and applied. Instead of the conventional one source of seismic waves, two sources were used separated by a system of paravanes and booms to allow collection of two seismic lines in one pass of the vessel. This reduced the cost of data collection by approximately 40%.
The center of the two sources were 75 m. apart the number of airguns used per source array was 17 with a volume of 2125 c.in. per arrays. The streamer was positioned in the middle between thw two source arrays. Its length was 2400 m. (96 channels) and the near offset was approx. 200 m. from each array. The variation of this length never exceeded 1-2 m.
Generally the performance of the above innovation system (novel technique) was problem-free and proved to be a real time-money saver.
As a whole the processing package that applied in the processing of the 3-D data is considered to be at the state of the art level. One of the applied techniques though sold be especially emphasised because it greatly reduces the cost of the 3-D survey. This is the Line Interpolation technique.
For the interpolation to be applied, the stacked data were first migrated in-line. Then the data were recorded into cross-lines and interpolated to a line spacing of 18.5 m. using Contractor's Intelligent Interpolation programme. After the interpolation the data were finally migrated in the cross-line direction. Interpolation generates a new trace between every two existing traces.
The interpolation technique was essential to reduce the actual shot line spacing from 37.5 m. to 18.5 m. allowing the high desired resolution to the final product by preserving higher frequencies ranging in this case from 37 to 54 Hz for cross-line dips ranging from 30 to 20 degrees compared to 26-28 Hz max. frequency preserved for non-interpolated data.
This novel technique generates additional lines for input the 3-D migration process that otherwise should be actually shot. The two to one interpolation represents 50% cost saving from lines not required to be actually shot. At the same time the max. frequency preserved in 3-D Migration were increased by 50%.

Programm/Programme

ENG-HYDROCARB 1C - Programme (EEC) of Community projects in the hydrocarbons sector, 1973-1985

Thema/Themen

1.4 - GEOPHYSICS AND PROSPECTING

Aufforderung zur Vorschlagseinreichung

Data not available

Finanzierungsplan

DEM - Demonstration contracts

Koordinator

PUBLIC PETROLEUM CORPORATION

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