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ABSTRACT: Case Study On Source Rock Deposition And Preservation - NW Borneo Deep Water Areas

Anuar, Azlina, and Abdul Jalil Muhamad
PETRONAS Research & Scientific Services Sdn Bhd, 43000 Kajang, Selangor Darul Ehsan, Malaysia

Deep-marine sediments of offshore NW Sabah show comparable hydrogen contents with middle-outer neritic sediments, while visual kerogen typing indicates an increase in hydrogen-rich liptinitic organic matter. This phenomenon is attributed to the narrowness of the offshore Sabah shelf which permitted the transportation and preservation of land plant-derived organic matter in deep-marine areas via slumpings and turbidity flows. Preferential-segregation may also contribute to the presence of liptinitic organic matter in the deep-marine environments.

Conversely, in the Baram Delta, hydrogen index values steadily decrease towards the distal areas. As organic matter deposition and preservation mainly occurred contemporaneously with deltaic progradation, the effect of preferential segregation is not as obvious here. It is expected that delta toe marine shales with terrigenous liptinitic organic matter contribution would represent the main hydrocarbon source rocks in the deep-water Block E area.

As such, land plant-derived source rocks are expected to exist in the Block G/H deep-water areas offshore Sabah, albeit more detailed work need to be undertaken to assess their volumetric significance. The recent gas condensates and minor oil discoveries in the Kebabangan and Kamunsu wells, however, attest to the generative capacity of the source rocks in that region. The Tembungo field is currently producing oil and gas generated by land plant-derived organic matter from turbidite reservoir sands. Towards the Sarawak deep-water areas, it is likely that terrigenous organic matter may prove to be less significant as hydrocarbon source rocks. Abolins & Scherer (1999) reported a marine origin for the hydrocarbons discovered in the B11 and B12 wells.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia