--> ABSTRACT: Chemostratigraphy Analysis of the Albian Age Sequence Orange Basin, Offshore South Africa, by Ngek, Nofu C.; #90142 (2012)

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Chemostratigraphy Analysis of the Albian Age Sequence Orange Basin, Offshore South Africa

Ngek, Nofu C.*1
(1) Earth Sciences, University of the Western Cape South Africa, Stellenbosch, South Africa.

Significant amount of the world’s hydrocarbon reservoirs often occur in sequences that have very poor stratigraphic control. The Albian age sandstones of block 2C Orange basin offshore, South Africa constitutes part of the important hydrocarbon bearing oil and gas play. The Orange Basin is a host of two fields with multi-trillion cubic feet potential natural gas reserves namely the Ibhubesi and the Kudu gas fields. So far only 38 wells have been drilled, hence, the Orange Basin remains under explored despite it’s very sizeable oil and gas potential, and recently it has become an area of research since it is believed to host hydrocarbons.

Correlation of the Albian age sequence is believed to be complicated due to barren and poor biostratigraphic sequences which makes stratigraphic and reservoir interwell correlations difficult. Hence reservoir quality management and construction of 3D stratigraphic models become problematic. Correlation between wells can become extremely difficult, and traditional correlation techniques such as lithostratigraphy and biostratigraphy can prove to be inadequate.

The study area, the Orange basin, is located offshore South Africa and is largely under-explored. It was formed during the late Jurassic to early Cretaceous periods due to the Gondwana breakup of the African and South American plates.

The main aim of this research work is to utilize chemostratigraphy technique to produce a chemostratigraphic correlation framework for the Orange Basin( block 2c) in order to facilitate stratigraphic, reservoir interwell correlation and reservoir quality management, which would be used as components for the construction of a 3D stratigraphic model of the Orange Basin South Africa. The objectives of this research are to; geochemically characterize the wells, deduce provenance of the rock types, paleo-weathering, paleo-climate , identify similar interwell geochemical indices and use the similar interwell geochemical indices to produce a chemostratigraphic framework, and to ultimately ascertain the uniqueness in the use of chemostratigraphy as a valuable preferred correlation tool to optimize reservoir performance and stratigraphic modelling.

A chemostratigraphy study was applied on one hundred and eleven cores and cuttings obtained from four wells, A-H1,K-A2 K-B1and K-E1 selected from the Albian age . Wireline logs (gamma ray, sonic, resistivity, caliper, neutron and density logs) were used to delineate lithology and reservoir zones after adequate depth shifting and environmental corrections were done. One hundred and eleven samples (core and ditch cuttings) were delineated based on the log signatures responses. They were made up of sandstones, siltstones and shales. Ten reservoirs zones were identified from the four wells which could probably contain hydrocarbons: Well A-H1 had five reservoir zones with reservoir units ranging from 2780.07-2837.99m , 3288.97-333.05m ,33404.04-3435.05m, 3504.05-3558m and 3750-3870m, three reservoir zones from Well K-A2 with depths ranging from: 3006.92-3039.90m, 3056.24-3084.97m and 3087-3120.2m and two reservoir zones from Well K-E1 with depths ranging from 3743.89-3749.85m and 3870.17-3875.59m .

XRD analysis was carried out on twenty one samples from the delineated reservoir units; eleven samples from well A-H1, four from well K-A2 and three from well K-E1. It identified the minerals present in samples and generally, the mineralogy of the analysed samples from the same well was similar with the following minerals: Quartz, Calcite, Muscovite, pyrite, Albite. The clay minerals present were Kaolinite, montmorillonite and chlorite. Quartz was the dominant mineral and it was found to be present in most of the sampled intervals.

Other than XRD ,Scanning electron microscopy provided information about the morphology, mineral composition, distribution of the neoformed authigenic clay minerals in pore spaces of the sandstones, mechanically infiltrated clays, transformational clays. The SEM results for the Wells show the presence of chlorite, montmorillionite and kaolinite. Kaolintie exhibits a reduction of porosity and not permeability. Montmorillonite reduces permeability and no effect on porosity, while chlorite reduces both porosity and permeability in the pore spaces in the reservoir zones. Based on the clay type, their abundance and distribution in the pore spaces of the reservoir, the reservoir quality of well K-A2 may be much more better than that of all the other wells marked by the presence of more kaolinite and less chlorite and smectite. The reservoir quality of Well K-A2 is better than that of Well K-E1 and the reservoir quality of Well KE-1 could be the least marked by abundant distribution of montmorillonite and chlorite in the pore spaces which reduces both porosity and permeability.

XRF analysis led to the identification of twenty seven elements, twelve major and fifteen trace elements and multivariate statistical analysis (cluster, discriminant function, factor and principal analysis) were used to process the XRF results to obtain similar interwell geochemical indices, provenance of the rock types, paleoweathering and paleoclimate.

Cluster analysis classified and grouped the samples based on similar chemical compositions. Three cluster groups were obtained. Group one made up of nineteen samples, group two made up of forty eight samples and group three made up of forty four samples. These samples were sandstones, siltstones and shale’s respectively.

Discriminant function analysis tested the existence of the clustered groupings, geochemically distinguished and characterized the groups .The groups were found to be 97.3% different from each other. Group one( Sandstones) were characterized by: SiO2 Na2O,CaO and MnO, group two(Siltstones) were characterized by:K2O, Al2O3, MgO and TiO2 and group three(Shales) were characterized by :Fe2O3, SO3,LOI and P2O5.

Zr/Ti ratios were used to determine provenance of rock types from the three clustered and discriminated groups. The rocks in group one have an average low Zr/Ti ratio of 0.0342 and according to (Ewart, 1982), a Zr/Ti ratio of 0.034 corresponds to andesites. Hence the rocks of group one could be sourced Andean volcanic rock more precisely from andesite’s. The rocks in group two have an average lower Zr/Ti ratio of 0.0218. They could originate from basic igneous rocks of Andean volcanic origin and more precisely from basalts. In (Ewart,1982) ratios between 0.024 corresponds to basalts and the ratio of group two rocks(Zr/Ti 0.0218) is close to the value; so we assume that they originate from basalts. The rocks in group three exhibit an average lower Zr/Ti ratio of 0.278. According to Ewart, in (1982) Zr/Ti ratios between 0.024 originates from basic igneous rock precisely basalts. The obtained Zr/Ti ratio for group three almost correlates to that value. Hence, we assume the provenance of the rocks is from basalts.

The chemical index of alteration (CIA) provided information about the chemical weathering processes which were controlled by the climate. They CIA values range of group one rock type(sandstones) ranges from 37% to 71% with an average alteration value of 60.60%.This indicates the rock samples have undergone low to moderate chemical weathering and are likely to have been formed in cold/arid climates (Nesbitt and Young, 1982). The CIA values of group two rock type(siltstones)ranges from 68% to 74.4% with an average alteration value of 73.8.60%.This indicates the rock samples have undergone moderate to high chemical weathering and are likely to have been formed in hot/humid climates (Nesbitt and Young, 1982). The CIA values of group three rock type(shales) ranges from 64% to 74%, with an average alteration value of 70.8.21%.This indicates the rock samples have undergone moderate to high chemical weathering and are likely to have been formed in hot/humid climates (Nesbitt and Young, 1982).

Factor analysis led to the identification of similar interwell element associations. Two element associations of major elements were common among the wells, SiO2 and Fe2O3, Mno, P2O5 and two trace element associations were common among the studied wells Zr ,Mo and Y,Ni, Rb.

The geochemical profiles enabled identification of similar interwell element enrichment, depletions units and significant similar interwell stratigraphic geochemical trends were recognized from which Chemostratigraphic zonations were developed from immobile element ratios; SiO2/Al2O3, Fe2O3,MnO, TiO2/K2O, Al2O3/K2O for major elements and Zr/Rb, Nb/Rb, Y/Zr and Ti/Zr.

The obtained chemostratigraphy zonations were compared with the biostratigraphy zonations of the same Albian age sequence and chemostratigraphy technique was able to subdivide and correlate the barren and poor biostratigraphy sequences which were proved to be inadequate by biostratigraphy technique.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California