An integrated study of the early cretaceous (Valanginian) reservoir from the Gamtoos Basin, offshore South Africa with special reference to seismic facies, formation evaluation and static reservoir modeling

The prediction of depositional environments and reconstruction of the reservoir geometry is of great significance in hydrocarbon exploration. Sedimentary facies distribution within a sequence is controlled by the environment of deposition. The analysis of facies within a basins based on seismic studies has become increasingly important in hydrocarbon exploration and is being used by most of the oil companies. The present study aims to determine and contribute to a better understanding of the depositional sequences by analysing information from the six drilled wells Ha-N1, Ha-I1, Ha-G1, Ha-A1, Ha-K1 and Ha-B2 from the north to the south for the Early Cretaceous Valanginian age group of sediments. A comprehensive interpretation of the model by Vail, et al., (1977) entails configuration and classification characteristics of seismic data reflection termination patterns, description of the seismic sequence, and seismic facies analysis of the sedimentary sequences. This was tied with well-logs (GR-ray) signatures from the wells and application of 3D-stochastic lithofacies model, were applied in this study. The depositional sequence was mapped and interpreted on seismic-reflection data for the J1horizon trending from north to south across the drilled wells in the basin. The interpretation configuration features are associated with seismic data reflection terminations patterns, according to the model by Vail, (1977) & Mitchum et al., (1977) for seismic sequence and seismic facies analysis. These are calibrated with well-logs signatures across the drilled wells from the north to the south of the study area indicating a mass flooding depositional sequence belonging to Valanginian age, deposited as channel-fill, submarine canyon, shallow marineto deep marine off continental shelf, submarine fan lobe and basin floor depositional environments. The sediments around the study area were deposited in a shelf to deep marine environment. The 3D stochastic lithofacies model derived from the surfaces attribute-depth-thickness map of the Valanginian depositional sequence revealed the facies and channels of a distributary depositional sequence pattern across the wells in the study area. The lithofacies model http://etd.uwc. ac.za/ 105 exposed the effect of the erosional events that created hiatuses from the north to the south during this period. The Valanginian marine flooding of high energy density turbidites affected the basin from northeast to southeast of the study area, leading to massive sandstone deposits and intermittent siltstones with minor intercalated shale deposits. In the area between the northwest and the southern part massive siltand shale deposits with an intermittent sandstone lithofacies of low energy turbidite channels was encountered. The northern part is generally dominated by shale with intercalated sandstones and minor silt. Keywords; Depositional environments, Lithofacies, Sandstone, Silt, Shale, Turbidite, Well-logs signatures, Shallow marine, Deep marine, Seismic sequence and Seismic facies. http://etd.uwc. ac.za/ 106 5.1 Seismic Interpretation 5.2 Introduction to Seismic Interpretation This chapter presents the seismic interpretation based on seismic sequence and seismic facies analyses for the depositional sequence top of P1-to-J1-horizons (Valanginian section) of the Gamtoos Basin and to describe the depositional environment of the sequence. The framework of the seismic analysis and interpretation begins with the seismic sequence analysis which is again based on the depositional sequence identified on a seismic section (Mitchum Jr, et al., 1977), by recognition and descriptions of the surface discontinuities, which are identified and interpreted in orderly patterns of reflection terminations along the discontinuity surfaces. This is followed by seismic facies associations, which are analysed and interpreted on the basis of configurations of the seismic reflections such as, amplitude, continuity, frequency and the external geometry reflection. This was followed by tying the interpretations among all lines to obtain traces on similar surfaces of different lines, either parallel to or across each other. This was connected further into the study area to trace the surface in a larger possible area. Some of the surface lines are of regional significance passing through wells, while others are local lines passing through a particular well in some section of the study area. The 2D regional extent seismic line (Ha75-12) shot from North-northeast (NNE) to South south-west (SSW), line (Ha87-047) shot from Northeast (NE)-to-Southwest (SW), cut across the wells Ha-G1 and Ha-A1 in the northern and southern part of the study area respectively. While the 2D-seismic lines Ha85-005 (NNE-NNW), line Ha82-12 (N-to-E), line Ha82-14(Sto-E), and line Ha76-031(E-to-W) were shot locally in the Northern and Southern part of the study area and cut across wells Ha-B2, Ha-I1,Ha-K1, Ha-N1 respectively. All these lines were used for the interpretation of the seismic sequence and seismic facies analysis and to examine the lateral changes patterns of the sedimentary succession across the study area. For a detail interpretation understanding of the depositional environment of the sedimentary succession, the information collected from the seismic facies analyses were calibrated with electrical well logs available from the wells that are drilled through the Valanginian succession (JI horizon). However, for the wells where such logs are not available the interpretations were carried out on their seismic reflection attributes, centred on the internal reflection configuration, the three-dimensional external geometry form, that is associated http://etd.uwc. ac.za/ 107 with the facies units, using simple techniques as explained by Mitchum Jr, et al. (1977a). Figure 5.1 shows the map window of the 2D-seismic lines across the wells for the interpretation of the seismic sequence and seismic facies analysis. Figure 5. 1: Map window of the 2D-seismic lines across the wells utilized for the interpretation of the Seismic sequence and Seismic facies analysis (Seismic stratigraphy analysis). 5.2. 1 Seismic Sequence Analysis and Interpretation of Results 5.2. 1.