--> Abstract: The Lost Hills Lower Brown Shale: A Diagenetic-Stratigraphic Trap in a Miocene Diatomite Reservoir, San Joaquin Basin, California, by Michael S. Clark and Victor Pusca; #90039 (2005)

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The Lost Hills Lower Brown Shale: A Diagenetic-Stratigraphic Trap in a Miocene Diatomite Reservoir, San Joaquin Basin, California

Michael S. Clark and Victor Pusca
ChevronTexaco, Bakersfield, CA

The Lower Brown Shale (LBS) play on the south flank of Lost Hills anticline in the San Joaquin basin, California produces gas where a Miocene channel-levee complex intersects the diagenetic transition where diatomite in the Opal CT silica phase (porcellanite) transforms to brittle Quartz (chert). Although the levees produces some gas, most production derives from a 1-km wide channel containing 1 to 5 sandy diatomite beds, each <9 m (<30 ft) thick and separated by sand-poor diatomites. The sand-rich beds in logs and core stand out as high-resistivity (<1.4 ohm-m) intervals, called “lobes”, containing, thin (<10 cm thick), ripple-laminated sandstones. Correlation of lobes to sonic travel times <150 msec indicates fracturing, whereas sand-poor beds between lobes are unfractured. Interestingly, the “interlobe” diatomites are Opal CT, whereas the lobes are Quartz phase. Apparently, sand grains facilitate the CT-Quartz transformation at Lost Hills at burial depths below 1.3-1.4 km (4,000-4,500 ft). By contrast, sand-poor diatomites remain Opal CT at the same depths. Most likely, the entire LBS interval contains gas, but the quartz-phase rocks fracture easier. Thus, where the channel-levee complex intersects the CT-Quartz transition, sandy diatomites in both channel and levee facies produce, provided these beds are fracture stimulated. Previously considered a secondary target, the LBS diagenetic trap is now the primary completion objective at south Lost Hills. Because the CT-Quartz transition extends around the basin perimeter, similar traps elsewhere in this basin are likely, as well as in other diatomite basins, such as the Sakhalin of easternmost Russia.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005