Tight Gas Reservoirs in the West Crocker Turbidites: A Frontier Play Offshore Northwest Borneo

Deepwater exploration offshore Borneo has mainly targeted Neogene clasitc sediments, while deeper reservoirs may as well provide hydrocarbon accumulations. The West Crocker Formation, with a thickness of at least 1,000 meters, has excellent exposures onshore Sabah. These Oligocene turbidites extend offshore below the Neogene sediments. We studied these rocks at several localities around Kota Kinabalu with the aim of assessing their potential hydrocarbon yield. The West Crocker sediments are some of the most sand-rich turbidites known in the world with thick sandstone layers (each layer ranging from tens of centimeters to a couple of meters). Samples collected from the sandstone show quartz content of over 70% and porosity of 8-15%. The whole sequence has been dismembered by reverse faults trending NNE-SSW and dipping east. We consider these sandstone units as potential tight gas reservoirs offshore Sabah. The source rocks are characterized by the Trusmadi Shale (dark grey argillite) underlying the West Crocker Formation as well as the organic-rich shale units (each tens of centimeters in thickness) interbedded with the turbidite sandstone. Total organic carbon content measured on the shale samples range from 0.1 to over 5%. Rock Eval (pyrolysis) Tmax for the same samples are 470-500 degrees Celsius and vitrinite reflectance values are 1.3 to 2.0%. These data indicate the source rock to be over-matured for oil but within the wet gas and dry gas generation window. The Upper Shale Unit of the West Crocker Formation may act as an effective cap rock atop the Lower Sandstone Unit. These shale units have very low porosity and permeability values. In addition, reverse faults containing fault gouge (with porosity of less than 5% and permeability of less than 0.001 millidarcy) also play an important role in lateral sealing and reservoir compartmentalization.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California