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COMPUTER APPLICATION AND STATICS COMPUTATION FOR EFFECTIVE FIELD DATA ACQUISITION IN THE NIGER DELTA
TABLE OF CONTENT
List of figures
List of tables
Table of content
Chapter Four: data computation, analysis, results and summary
Background on automated computer applications
In the Niger Delta, South of Nigeria, 3-D seismic reflection statics ad different points in a typical common-depth-point (CDP) gather can differ from one another by 60 milliseconds because of variation in the thickness of the weathered zone that extends deeper than 63m on the relative gentle topography, onshore to the swamps and offshore, were the problems of weathering is very erratic. Velocities of more than 900 m/sec were recorded in the swampy areas and average velocity or 1387 m/sec was also recorded with uphole shooting in this area.
Recording was done at selected areas of the Niger Delta: central South of the Delta (Syverline channel prosect). Awoba, in the complex creek and Afam/Isimiri/Ebubu/Korokopro (OML11).
Automated procedure, using data from manually picked first breaks from the intercept and the plus-minus method in conjunction with previous 3-D uphole/refraction data from computing statics where introduced to account for velocities, delay times and thickness variations as well as localised topographic effects.
Results obtained were of acceptable standard especially with the automated filed procedures.
Extreme variation in p – wave travel time in the zone of weathering degrades the quality of seismic reflection data obtained in the South eastern part of Nigeria.
The application of static correction become imperative to problems caused in this region by topographic relief of about 10m local variation in the depth to the water table of more than 50, the low and variable p-ware velocity in the vadose zone (400 – 800m/sec) and the presence of a weathered zone that reaches depths of more than 60m.
Static corrections are applied to seismic data to eliminate the effects of variation in elevation weathering thickness and weathering velocity. The underlying principle is to determine the reflection arrival times which would have been observed if all shots and receiver position were on an agreed datum below which all matter is consolidated (fig 1-1)
The large differences between the weathered layer velocities and those of the underlying consolidated layer, result in signals travelling approximately vertically through the weathering, unless the depth of the reflector is very small and shooting distance very long (fig 1.2)
The corrections for the variation in the near surface are therefore assumed to be time invariant (static) and surface consistent.
Computation of weathering correction may not align reflection signals perfectly (fig 1-3). Hence statistical alignment of reflection signals is done through automated procedurrews with computers, fro residuals.
Field statics computations is the primary concern of this work. This work compares several methods which include, the uphole/refraction methods, the intercept method, the ABC method (Coruh, 1975) and the plus – minus method to computer assisted processing methods such as gardner – Lecalerc, statics velocity and time evaluation (SVELTE) methods, and Governor (Micromax and Geomicro) method of CGG and Western Geophysical Nigeria Ltd. Packages.
Data use to test these static corrections methods were obtained from a 200 km segment of a dynamite 3D-seismic reflection survey on the Agbada prospect (fig, 1-4) and the Afam/Isimiri/Ebubu/Korokoro area (fig 1.5) all carried out by CGG Nig. Ltd. Other data used were derived from the 3D-seisic reflection survey in the Syverline channel Awoba area prospect (fig. 1-5 and fig. 1-6) undertaken by Western Geophysical Nig. Ltd. As part of this survey, uphole/refraction seismic measurements were made at several points on the Agbada and Afam /Isimiri/Ebubu/Korokoro prospects making a profile of 150km to determine velocity structures in the weathering zone along the various seismic reflection lines (shot point – Receiver lines) static corrections calculated from these data were used as standard values from judging the accuracy of corrections obtained by manually picking first breaks (intercept and plus-minus methods)
The area of study is th Niger Delta, in which selected areas were chosen for data source (fig 1-6)> the first palce, sylverline channel, lies in the Niger River Delta of South central Nigeria. The area is under the legal jurisdiction of the Degema local government, Bonny local Government as well as the Okrika, Oyigbo Tai-eleme local Government, all in Rivers State. The survey area is suited between latitude 4 degrees 32 minutes North and longitude 7 degrees 02 minutes East. The prospect is named after a shell production field which is centred within 3D survey block. The sylverline channel bisects the survey block from the point where it leaves the New Calabar River to the south east corner of the block in a north-west to south-east direction. The survey block is also bisected by the New Calabar River in a North-south direction. Various other smaller creeks run through the area. The most important of these being the Krakrama creek at the western side of the block.
Most of the parts of the prospect are flat, low lying terrain consisting of mangrove swamps throughout.
The Awoba Area 3-D project also lies in flat low lying terrain consisting of mainly mangrove swamps. The top soil in these two areas is made of dead organic materials (peat) and dark mud in some areas. These areas are mainly sedimentary fills from the hinterland of Nigeria. The sediments are mainly sands and tense for clay in between (Benin formation). In this area where 63m of hole was drilled, the succession of sediments was generally represented as gross coarsening upward sequence of major regressive cycle (sdhort and Stauble, 1967; Frankl and Cordry, 1967)
As a result of the above, the sequence wit marine clays, is not only observed laterally, but is also encountered vertically in the Niger Delta (Frankl and Cordry, 1967) (fig 1-7)
The Agbada prosepect in the South-east central part of the Niger Delta and the Afam/Isimiri/Ebubu/Korokoro area in the North eastern axis of the area are onshre hence the areas are hill gentle relief. The sediments are mainly sands with silty clay to gravely or very sandy terrestrial deposits which change seaward to dandy and clayey fluviatile and mairne sediments which in turn change to marine clays (Frankl and Cordry, 1967)
The study of these selected points in the Niger Delta are very representative, hence they can be used to correlate and determine the weathering characteristics of the whole Niger delta.
1.3 THE NIGER DELTA AND ITS ATTRIBUTES
THE Niger Delta is located in the central southern part of Nigeria on the west coast of Africa. It is one of the world’s largest deltas.
The sediment from the hinterland of Nigeria and environs accumulate in the indentation caused by the separation of South America from Africa which is about 200km wide (Reyment, 1972). This area started filling up with sediments in the Turonain and gradually bulge out into the Gulf of Guinea, at the mouth of the Niger, Benue and Cross River drainage system (Dr. Ukaigwe 1992 – personal communication). Catchment area covers over a million square kilometres of savannah covered low-land, and stretching about 300km from the apex to the north and occupying an area of about 140,000 sq km (fig 1-8) the delta consists of a sedimentary prism of the order of 12km over all thickness deposited initially on continental crust and later on ocean crust (Burke et al, 1972), whiteman, 1982, knox and Omotsola 1989).
Sharp changes in topographic relief are presently associated with shale ridges and intervening sediment accumulation giving gradients of upto 10.
The Benin formation (which is also called coastal Palin sands) are milky-white to white yellow, fine to very pebbly sst. Toward the coast they are represented by the coastal plain stands with occasional clay beds and lignite seams on-shore. (fig 1-7 and fig 1-9).
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