ABSTRACT: Distribution and Maturation of Source Rocks in Brunei Darussalam

SCHERER, MICHAEL, and RAMLEE HITAM, Brunei Shell Petroleum

Hydrocarbon source rocks in Brunei were deposited in a prograding Tertiary delta setting. Although of land plant origin, these source rocks show a variety of types including resins, herbaceous material, structureless organic and woody gas-prone matter. Marine kerogenous shales have not been encountered.

Source rocks occur mainly in coastal plain and inner to middle neritic sediments. In the coastal plain setting coaly source rocks prevail in low pH (<7) fresh water peat swamps. No discrete, rich coaly layers of mappable extent have as yet been identified. In the inner to middle neritic setting (pH 8), bacterially reworked and partly structureless land plant matter is enriched in mudstones. Dispersed, reworked coaly material is still abundant in silts and sands, but the organic matter is often oxydized.

Total organic carbon (TOC) contents and yields show distinct variations from one environmental setting to the other. Coastal plain sediments vary strongly in TOC content and may have a potentially high gas yield. In mudstones of inner and middle neritic origin, average TOCs of 4% are commonly present in mudstones with considerable cumulative thickness (individual bed thicknesses of 5 to 10 m). Potential yields are 15-20 kg/ton of oil. Outer neritic to bathyal mudstones have low TOCs (<1%) and probably contribute little to hydrocarbon charge.

Although difficult to map, an approximation of the lateral extent of the inner to middle neritic prolific source rock sequences was attempted from paleogeographic maps. The volume of source rocks obtained in this way was then used to calculate charge potential within effective drainage areas.

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)