39 Journal of Petroleum Geology, Vol. 28 (1), January 2005, pp 39 - 48 1 Dept of Chemistry, Obafemi Awolowo University, Ile-Ife, Nigeria. 2 Dept of Geology, Obafemi Awolowo University, Ile-Ife, Nigeria. 3 Humble Geochemical Services, Humble, Texas, USA. 4 Dept of Chemistry, University of Ibadan, Ibadan, Nigeria. *corresponding author, email: stevesanya@yahoo.com A RE-APPRAISAL OF THE APPLICATION OF ROCK-EVAL PYROLYSIS TO SOURCE ROCK STUDIES IN THE NIGER DELTA A. Akinlua * 1 ,T. R. Ajayi 2 , D. M. Jarvie 3 and B. B. Adeleke 4 Thirty four shale samples from the Tertiary Agbada Formation were analysed for TOC and Rock-Eval pyrolysis parameters in order to evaluate the effect of oil-based mud contamination on source-rock characterization. The samples were obtained from five wells in the offshore Niger Delta over a depth range of 5,460ft to 11,580ft.The results indicated that the raw (unextracted) samples were dominated by Type III kerogen. However, after extraction, both Types II/III and III kerogen were identified, consistent with previous studies. These results demonstrate that it is essential that shale samples should be extracted prior to TOC and Rock-Eval pyrolysis for accurate source-rock evaluation. INTRODUCTION Total organic carbon (TOC) analysis and Rock-Eval pyrolysis are often used to assess the abundance and quality of organic matter in a potential source rock. These techniques are rapid and inexpensive. Optical methods are also used for source rock characterization but are comparatively expensive and require more sample preparation time. Thus they are usually only applied on a limited number of samples selected based on TOC and pyrolysis results. This also applies to elemental analyses of kerogen (Tissot and Welte, 1984). Previous studies of the Niger Delta (e.g. Udo et al., 1986; Nwachukwu and Chukwura, 1986; Bustin, 1988; Akaegbobi, 2000; Olaleye et al., 2000) have classified kerogen there mainly as Type III based on TOC and Rock-Eval pyrolysis, which implies gas- prone organic matter (Tissot and Welte, 1984; Stoddart et al., 1995; Ekweozor et al., 2000). These studies have not drawn attention to the presence of oil-prone source rocks, and yet both crude oil and natural gas are produced in the Niger Delta. This could be due either to the absence of oil-prone source rock intervals, or to the reduction of hydrogen indices in the samples analyzed due to the presence of drilling fluid additives, causing the oil-generating potential to be underestimated. A mineral matrix effect may also account for HI suppression (Udo et al., 1986). Studies based on biomarkers and the results of pyrolysis-gas chromatography have indicated the presence of mixed Type II/III kerogen in the Niger Delta (Ekweozor et al ., 1979; Udo et al., 1992; Nwachukwu et al., 1995; Akinlua, 1997; Akinlua et al., 2000). This study was undertaken to investigate the influence of drilling mud additives on Rock-Eval pyrolysis data, and consequently on the classification of organic matter in the Niger Delta. Geological Background The geology of the Tertiary Niger Delta is well known (Short and Stauble, 1967; Weber and Daukoru, 1975; Evamy et al., 1978; Ekweozor and Daukoru, 1984; Lambert-Aikhionbare et al ., 1992) and is not discussed here in detail. The subsurface stratigraphy consists of the Akata Formation (a massive marine shale) overlain by the Agbada Formation (composed of paralic sandstones and shales), which is in turn overlain by the massive sandstones of the Benin Formation (Evamy et al., 1978). Reservoir rocks for