TOPICAL TROPICAL NON-MARINE DEEP WATER DELTAIC CHARGE SYSTEMS IN SE ASIA. A MODEL TO EXPLAIN WHY SOME ARE OILY AND SOME ARE NOT

摘要

Recent deep water oil discoveries in the Mahakam and Baram Deltas have been made beyond the known limits of the coastal plain source rocks that generated the prolific inboard accumulations. The geochemical signatures of the inboard and outboard oil families are similar, however, with both showing strong evidence of organic matter derived primarily from land plants. Two plausible models involving mega-slumps and/or turbidites have been proposed in the literature to explain how the terrestrial source material was transported into deep water. Equally intriguing are recent failed tests in other deep water basins (e.g. West Baram Delta) and shelf settings such as, the Sandakan Basin. In addition, portions of the Mahakam and Baram deepwater plays are gas-prone yet are immediately adjacent to oil discoveries in contemporaneous reservoirs. The pattern of discoveries indicates that the coastal plain source rocks are not developed in all Tertiary deltas and that deep water settings have both source presence and phase risk. This paper examines the petroleum systems of SE Asia's Tertiary deltas and associated deep water basins and proposes a model to explain the distribution of oil accumulations. The model has three key components:- ⅰ) "delta top ponding" - the mechanism that controls deposition of thick oil prone source intervals; ⅱ) eustatically and tectonically controlled transportation of source material into some deep water environments and ⅲ) preferential accumulation of oil by leakage of gas up faults or through the top seal. This "three element model" is used here to explain the observed distribution of hydrocarbons in several proven Australasian basins. Its predictive capability in some topical unproven deepwater areas of SE Asia is also discussed.