ABSTRACT: Turbidite and Deepwater Depositional Systems of Borneo: Foredeep Slope and Basin Floor Fan Systems

CREVELLO, PAUL D. , Petrex-Asia Reservoir and Stratigraphy Consultants, Brunei, SE Asia

Turbidites are relatively new exploration targets in deep-water plays of Southeast Asia, viz. the Mahakam and Baram Deltas and NW Sabah Shelf, where new reservoir sands with sustainable high flow rates have been discovered. However, prediction and imaging of hydrocarbon-charged sands in complex structurally active sedimentary basins has led to a low number of total discoveries. Enhanced imaging of pay sands or predicting sand fairways is complicated by shifting receptacle basins and structural deformation of reservoirs; and challenged by detection of optimal reservoirs, continuity, and vertical connectivity of reservoir sand systems. Knowledge about the hinterland source area, shelf-staging area, sand influx, and distribution mechanisms are poorly constrained because of the complicated tectonic and prolonged turbidite basin history of Borneo.

Vast areas of Borneo persisted as deep-water turbidite systems throughout much of the Paleogene. The Rajang Turbidite Group, which forms the backbone of Borneo, was deposited in a foredeep basin during the Cretaceous to Eocene. Younger turbidite systems of the West Crocker, Temburong and Setap Formations occur in outcrop and offshore NW Borneo, and important hydrocarbon-bearing turbidite sequences occur in offshore regions of the Miocene- Pleistocene Kutei, Baram, and NW Sabah basins.

Two systems worthy of note were deposited in turbidite basins of Borneo: the West Crocker submarine fan and the Neogene turbidite systems; these range from middle–late Oligocene to middle Miocene and middle Miocene in outcrop to Pliocene-Pleistocene, respectively, in the offshore hydrocarbon provinces. These turbidite systems provide examples of slope canyon feeder systems, ponded slope basins, and basin floor "unconfined" systems.

The Crocker Formation is a "classic unconfined" foredeep basin-floor submarine fan complex. The fan was deposited in a foredeep trough that extended for several hundred kilometers along the Borneo trench. Sand-rich channel-sheet complexes exceed 300 m in thickness and the entire fan system extends over 12,000 sq km, rivaling in size and sand volume world-class submarine fan systems. Individual channel axis sands rarely exceed 3–5 m, while channel and sheet sands are amalgamated into 30–60 m-thick multistory sand complexes. Mud-rich levees are less evident, and the high net:gross supports a sand-rich system. The regional extent of the fan indicates an extensive complex of off-lapping, "unrestricted" channel-lobe fans. Uplift of Paleogene turbidite sands, like the Crocker, provided the source for much of the recycled sands of the Neogene turbidite systems.

The Neogene turbidite systems formed in ponded basins and unconfined basin floor fans around the peripheral basins of Borneo. These systems recorded clearer linkage between sedimentation and tectonics. Turbidite channel sands and lobes thin and onlap or are truncated along active seafloor structures, faults and shale diapirs. Depositional cycles contain mega-slumps, olistoliths, and debris flows alternating with channel and sheet/lobe sands. Individual channel and sheet sands rarely exceed 3 m while amalgamated multistory sands typically range between 10 and 30 m. Linkage with lowstand shelf-edge deltas and/or tectonic episodes with optimal reservoir sand quality, input and shelf bypass is recorded in the Neogene systems. Subsurface and outcrop examples and selected analog studies displaying 2-D and 3-D aspects of turbidite fan systems, especially reservoir elements and seismic and well-log facies attributes, will be used to elucidate the turbidite systems of Borneo.

AAPG Search and Discovery Article #90916@2001-2002 AAPG Distinguished Lectures