KUPECZ, JULIE A., ARCO Exploration and Production Technology, Plano, TX

ABSTRACT: Reservoir Potential of a Major Source Rock: Upper Triassic Shublik Formation, Prudhoe Bay, Alaska

The Upper Triassic Shublik Formation is a prolific source rock for North Slope hydrocarbons, containing an average total organic carbon (TOC) content near Prudhoe Bay of 5.6%. Approximately 32 billion barrels of Shublik-sourced oil are estimated to have migrated and accumulated in North Slope reservoirs located between the Colville and Canning Rivers, assuming a 10% migration and accumulation efficiency. Potential also exists within the Prudhoe Bay Field for the Shublik to be a producing reservoir.

Within the Prudhoe Bay Field, the Shublik has been subdivided into four zones, lettered from top to base, A through D. These zones comprise two shallowing-upward third-order sequences. Lithologically and mineralogically, the Shublik is complex, having carbonates (limestone, dolomite, siderite), phosphates, siliciclastics (shale, sandstone, siltstone), and accessory glauconite, pyrite, and kaolinite. Zone D is composed of fine-grained sandstone, but is thin and areally confined to the western part of the field. Zones A and C are composed of shaly limestones to calcareous silty shales, while Zone B is a phosphatic carbonate to phosphatic shale.

Porosity mapping of the Shublik suggests good reservoir potential within the Prudhoe Bay Field. Areas of highest porosity are interpreted to have been diagenetically enhanced due to the creation of moldic porosity and the dissolution enhancement of some fractures. The distribution of porosity is parallel to the northwest-southeast trending Lower Cetaceous Unconformity (LCU) and the east-west trending North Prudhoe Bay Fault Zone. Highest porosity values are concentrated near the intersection of the LCU and the North Prudhoe Fault. It is interpreted that corrosive fluids moved along the unconformity and the fault zone and dissolved carbonate allochems, thus creating moldic porosity. Estimated oil in place for the Shublik may be as much as 500 million barrels.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.