British Journal of Science 42 January 2012, Vol. 3 (1)

Transcription

1 British Journal of Science 42 The Development of the Geologic Model of the Gongola Basin Depositional Sequence Using Lithofacies Map E.E.Epuh*, D. O.Olorode** P.C. Nwilo, C. U Ezeigbo * Department of Surveying and Geoinformatics, University of Lagos ** Physics Department, University of Lagos Abstract Seven horizons representing the six stratigraphic units and the top basement of the basin were picked from the seismic sections for mapping and lithofacies analysis. The lithofacies parameters such as lithology thickness, interval and depth normalized interval velocities, density, reflection coefficients and acoustic impedances were determined using the sonic log, checkshot curves and the iterative depth algorithm (IDA) process. In order to obtain a subtle gradation of the lithofacies, they maps were produced using patterns. The various pattern observed on the maps show the areal distribution of the facies in each stratigraphic unit. The delineation of the maps helped in the development of a geologic model of the basin depositional sequence. The basin shows the progradation of sand towards the northwest part of the project area at the prospect horizon; with sand thickness and reservoir quality increasing in the southeast direction. This is as a result of the sand nourishment provided by the presence of Gaji and Kolmani River. The basin predominantly shows a combination of laterally and vertically gradational sequence with some parts also exhibiting the presence of intertonguing sequence. Keywords: Lithofacies, interval velocity, depth normalized interval velocity, progradation, Gongola basin 1.0 Introduction As subsurface mapping techniques develop further, it becomes conventional to examine not only the areal distribution of stratigraphic units beneath an unconformity, but also to examine the kinds of rock units beneath that unconformity. It is observed that no single number, symbol or code shown on a map can completely express all the compositional aspects of a stratigraphic section. The variety of rock types present, their repeated positions on the stratigraphic interval, and their varying thicknesses of the beds from top to bottom of the section represent complex interrelationships that require maps especially designed to bring out features of importance in a given study. In this study, lithofacies maps were produced with distinct pattern representing variations of rock compositions within a given stratigraphic unit. Facies map show the areal variation in every aspect of a stratigraphic unit. When the facies maps are based on lithological attributes, they are called lithofacies maps. Lithofacies are defined as the sum total of the lithological aspects of stratigraphic units in a particular place (Reijers, 1995). A lithofacies describes a rock unit in terms of a set of lithological features which are related to one or more depositional process ongoing in time. In this research, the lithofacies maps were used to define the upper and lower bed boundary in time at which the depositional process changed. 1.1 Geology of the Basin The area under investigation can be classified into two broad geological regions namely: the Basement Complex and the sedimentary formations.

2 British Journal of Science 43 The basement Complex outcrops predominantly in a narrow strip along northwestern sector of the project area. The parts where the basement complex outcropped are usually bare of soil or if soil is present at all, it is only very thin soil type with high pebble and gravel content. The soil is usually reddish and derived from the weathering of the basement. The vegetation on the soil is very sparse and consists of only stunted and toughened tree-lets.(sparks, 1972). The crystalline basement complex consists of scattered remnants of highly metamorphosed sedimentary rocks and diverse, predominantly granitic, plutonic masses collectively known as older granites (Carter et al, 1963). The stratigraphic sequence of the formations in the project area is essentially the same as that of the adjacent regions with the Bima sandstone unconformably overlying the Pre-Cambrian/Palaezoic crystalline basement complex. The Yolde formation overlies the Bima sandstone. Marine conditions persisted during Lower Turonian times, leading to the emplacement of Pindiga formation. The marine condition gradually gave way to deltaic and estuarine environment in which the Gombe sandstone was deposited. The Gombe sandstone outcropped in a few isolated locations in the project area. Shale and ironstone are other elements of the Gombe sandstone. The whole sequence is overlain by the tertiary sandstone of the Kerri-Kerri formation. The Benue Trough has been described by several authors as a rift structure whose evolution is linked to the opening of the South Atlantic (King, 1950, Cratchley and Jones, 1965, Wright, 1976, Whiteman, 1982). The structural history of this area commenced towards the beginning of the upper cretaceous with the rifting of the upper Benue Trough and the accumulation of considerable thickness of sediments. A number of depocentres have been identified within this NE-SW trending depression. The sub-basins in the northern half of the trough include Bornu, Gongola (Dukku-Bag). 2.0 Methodology The seismic sections used in this study include 15 crosslines and 8 inlines. The time data obtained from the seven horizons of the seismic sections were converted to sediment depth, interval and depth normalized interval velocities using the sonic log, checkshot curves and iterative depth algorithm (IDA) process. The results obtained were used in the computation of the reflection coefficients and acoustic impedance of the horizons. These entire results were exported to Arc Map software for the development of facies maps (with pattern) of the entire horizons. The geologic model of the depositional sequence was developed through the analysis of the lithofacies maps of the seven horizons. The sand/shale ratio within a given stratigraphic unit was established by computing the areal distribution of the lithofacies using the coordinates of points at the transition boundaries. The volume was not considered since the lithofacies share the same thickness within a stratigraphic unit. The stratigraphic profiles showing the depositional sequence of the basin was also developed. The study area is OPL 803/806/809 of the Shell Nigeria Exploration and Production Company (SNEPCO) concession in the Gongola basin. Fig. 1 shows the seismic base map of the study area while Fig 2, shows the seismic section of the crossline D-036 and its mapped horizons.

3 British Journal of Science 44 N L E G E N D S e is m ic Line s R. G a ji R. Ko lm a n i R ive r Figure 1: Seismic Base map (Source: SNEPCO)

4 British Journal of Science 45 HR 1 HR 2 HR 3 HR 4 HR 5 Top Basement HR 6 Fi g. 2: Line D-36 showing the horizons used for Lithofacies analysis 3.0 Results and Analysis 3.1 Results The lithofacies maps obtained from each horizon are shown in Figures 3 to 9 respectively. Each of the maps shows the variation and areal distribution of the lithofacies within a given stratigraphic unit. 3.2 Lithofacies Map Analyses From the lithofacies maps, the following are inferred: i. Horizon H1 This horizon shows lithofacies variations in all the project area. This is as a result of the influence of geomorphology. The lithology is indicated as the combination of shale and sandstone and this falls within the Kerri-Kerri stratigraphic formation. The sediment thickness is 500m (between the surface and depth of 500m). The sand/shale ratio is estimated as 7:3 within the unit. The lithofacies map of this horizon is shown in Fig 3

5 British Journal of Science 46 ii. Horizon H2 and H3 At these horizons, the lithofacies are uniform. They horizons show similar lithofacies variations because they have the same rate of sedimentation and cementation. The dominant lithology is shale which is shown in green colour with little presence of dolomite. These horizons fall within the Gombe and Pindiga stratigraphic formations. The sediment thickness is 1600m (between the depth of 500m and 2100m respectively). The sand/shale ratio is estimated as 9:1 within the unit. The lithofacies maps of these horizons are shown in Fig 4 and 5. iii. Horizon H4 At this horizon, the upper part of the project area, beyond the Gaji River (from inline D-047) shows area of shale dominant lithology. This is shown in green colour. Lead L-A closure which was identified in the time and depth structural maps disappears at this horizon because of the dominant presence of impermeable shale sediments. Lead L-B which lies in the Kolmani River shows the dominant presence of dolomite (gas sand). The gas sand is shown in white colour within the dolomite zone, while the dolomite is shown in brown colour. The variable patterns within the lead L-B lithofacies are as a result of differential sedimentation, cementation and compaction caused by the presence of Gaji and Kolmani Rivers. This also indicates the lateral variation in porosity and permeability of within the lead. This horizon lies within the Yolde stratigraphic formation. The sediment thickness is 600m (between the depths of 2100m and 2700m respectively). The sand/shale ratio is estimated as 4:6 within the unit. The lithofacies map of this horizon is shown in Fig 6. iv. Horizon H5 At this horizon, there is progradation of dolomite towards the northwest part of the project area with the progradation terminating at inline D-045. The lithology is identified as dolomite and shale. The horizon lies within the Bima stratigraphic formation The sediment thickness is 700m (between the depths of 2700m and 3400m respectively). The sand/shale ratio is estimated as 6:4 within the unit. The lithofacies map of this horizon is shown in Fig 7 v. Horizon H6 At horizon H6, the dolomite progradation has moved up to inline D-041. The lithofacies patterns show distinct segmentation caused by variable progradation towards the northwest part of the project area. The dolomite and shale lithological units are segmented because the sediments were distributed and accumulated with changing energy pattern. This horizon lies within the Bima stratigraphic formation. The sediment thickness is 1800m (between the depths of 3400m and 5200m respectively). The sand/shale ratio is estimated as 9:1 within the unit. The lithofacies map of this horizon is shown in Fig 8. vi. Top basement At this horizon, the lithofacies variation is erratic, and it covers the entire project area. This can be visualized using the erratic colour variation exhibited on the map. The lithology is identified as gneiss. The sediment depth is 5200m. The map is shown in Fig 9. The six stratigraphic sequences of the basin are shown in Fig 10.

6 British Journal of Science Depositional Sequence Analysis of the Leads Two leads L-A and L-B were recognized between horizon H4 and H6 in the time and depth structural maps. Their lithofacies analyses are discussed below. a. The Garin Habu Lead: The depositional pattern is one of thinning of sand from southeast to northwest part in a downlap fashion. These units thin in the same direction over a wide region without regard to the local structure. This shows a regional pattern involving landward edge of deposition, with the thinning caused mainly by starvation because of remoteness from the source (Rivers Kolmani and Gaji). The sediments are shale which is fine-grained sediments that lack permeability and are not of reservoir quality. The closures obtained in the structural (time and depth) maps vanished in the interval, depth normalized interval velocity and lithofacies maps. From this, it could be inferred that the structural closure in lead L-A is found to be a result of velocity gradient rather than hydrocarbon accumulation. b. The Kolmani River Lead: Further evaluation of the interval and depth normalized interval velocities and lithofacies map shows clearly the presence of gas sand. The depositional pattern is one of thickening of sand from northwest to southeast in a downlap fashion. These units thicken in the same direction over a wide region with regard to the local structure. This shows a regional pattern involving landward edge of deposition, with the thickening caused mainly by nourishment provided by its closeness to the source (Rivers Kolmani and Gaji). The sand sediments are coarse sediments (Dolomite) that have permeability and are of good reservoir quality. The gradual spread out of the divergent reflections as shown in the crosslines sections (Fig 2: D-036) in the dip directions indicates the areal variation in the rate of deposition, progressive tilting, differential basin subsidence and faulting which resulted in the gradual tilting of the basin in the eastern direction. The channel fill sand bars present in the lead L-B are generated by the fluvial process of Rivers Gaji and Kolmani with channel cut and then channel fill and valley fill which resulted in stream channels migrating laterally. The lateral accretion point bars deposits which fills the entire channel from the base to flood plain level gradually fills the latter. During successive floods by the Gaji and Kolmani Rivers, different lithologies were deposited which grade upward into flood plain ooze and yield the stratigraphy of the basin sequence beginning at the top. Based on the above depositional process, the channel fill sand bars found in the Gongola basin possess the following features as displayed on the crosslines seismic sections. a. Shape: It is observed that the channel fill sand bars have flat top with an asymmetric plano-convex base that is perpendicular to the paleodepositional strike. The axes of the grains are parallel to the channel axis. The sand thickness was at the expense of the overlying shale with thickness increasing toward the basin in the west-east direction. This can be seen using seismic sections from the cross lines. b. Internal structure: The horizontal trough stratification (constant thickness of horizontal strata is fairly common) within horizon H1 and H2 respectively. The basin is asymmetrically filled with thickness divergence toward axis. This is common because of the differential tilting and oscillations of the flood plain. The pre-compaction cut and fill shows no deformation of the lower time marker, however, compaction deformation of upper time marker is identified.

7 British Journal of Science 48 From the Lithofacies maps, the underlying geometry of the stratigraphic units and the pattern of their areal variation in lithology are easily visualized. Also, the lithofacies boundaries are easily classified and mappable. The inference obtained from the analysis of the lithofacies maps of each horizon led to the development of the geologic model of the depositional sequence of the basin which is shown in Fig 9. It shows that the progradation is towards the northwest of the project area. It also shows the decrease in grain size with an increase in the thickness of shale towards the direction of progradation. The reservoir quality increases towards the southeast with an increase in sand thickness. 4.0 Summary of Findings The application of patterns in this research has helped in the classification of lithofacies boundaries in terms of the following: i. Determination of boundaries at sharp lithologic contacts and laterally gradational sequence. ii. Determination of alternative boundaries in a vertically gradational sequence. iii. Determination of possible boundaries for laterally intertonguing sequence. iv. Determination and possible classification of parts of an intertonguing sequence. Also, in using the patterns as produced by the Arc Map software, the following risks were avoided: i. Description of the lithofacies using irrelevant details ii. Missing out critical parameters within the lithofacies. iii. Inconsistency in describing similar lithofacies in various parts of a stratigraphic unit and the entire project area. iv. The determination of the areal distribution and the transition zones of the lithofacies. 5.0 Conclusion It is clear that there is no unique seismic signature for each lithofacies and sedimentary environment. However, the application of pattern recognition in the delineation of lithofacies makes it easy to identify variations within a given stratigraphic unit. It also helps in mapping the lithofacies boundaries in three dimensions (3D) with the fourth attribute been the colour pattern (C) for distinction. This approach could be expressed as a 3D+C pattern. The integration of the 3D+C approach with the numbers, codes and symbols on a map will aid in a better definition of the lithofacies within a given stratigraphic unit.

8 British Journal of Science 49 Fig 3 : Horizon H1 Lithofacies Map showing the various sand and shale pattern

9 British Journal of Science 50 Fig 4: Horizon H2 Lithofacies Map showing the various sand and shale pattern

10 British Journal of Science 51 Fig 5: Horizon H3 Lithofacies Map showing the various sand and shale pattern

11 British Journal of Science 52 Shale Dolomite Kolmani River Gas Prospect Location Fig 6: Horizon H4 Lithofacies Map showing the various dolomite and shale pattern and the gas locations.

12 British Journal of Science 53 Kolmani River Prospect Fig 7: Horizon H5 Lithofacies Map showing the various dolomie and shale pattern

13 British Journal of Science 54 Fig 8: Horizon H6 Lithofacies Map showing the dolomite and shale with their variable energy distribution pattern. Fig 9: Top Basement Lithofacies Map showing the various Gneiss and shale pattern

14 British Journal of Science 55 H1 H2 H3 H4 H5 H6 Top Basement Fig. 10: Lithostratigraphic Sequence from H1 down to Top Basement) Depositional Strike N NW Grain size decreases Progradation Reservoir Quality increases Shale thickens Basement Sand thickens Gneiss/Granite

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