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ABSTRACT: Mixing of Biogenic Siliceous and Terrigenous Clastic Sediments: South Belridge Field and Previous HitBetaNext Hit Field, California

Daniel E. Schwartz

The intermixing and interbedding of biogenically derived siliceous sediment with terrigenous clastic sediment in reservoirs of upper Miocene age provides both reservoir rock and seal and influences productivity by affecting porosity and permeability. Miocene reservoirs commonly contain either biogenic-dominated cyclic diatomite, porcelanite, or chert (classic Monterey Formation) or clastic-dominated submarine fan sequences with interbedded or intermixed siliceous members of biogenic origin.

Biogenic-clastic cycles, 30-180 ft thick, at South Belridge field were formed by episodic influx of clastic sediment from distant submarine fans mixing with slowly accumulating diatomaceous ooze. The cycles consist of basal silt and pelletized massive diatomaceous mudstone, overlain by burrowed, faintly bedded clayey diatomite and topped by laminated diatomite. Cycle tops have higher porosity and permeability, lower grain density, and higher oil saturation than clay and silt-rich portions of the cycles.

Submarine fan sediments forming reservoirs at the Previous HitBetaTop field are comprised of interbedded sands and silts deposited in a channelized middle fan to outer fan setting. Individual turbidites display fining-upward sequences, with oil-bearing sands capped by wet micaceous silts. Average sands are moderately to poorly sorted, fine- to medium-grained arkosic arenites. Sands contain pore-filling carbonate and porcelaneous cement. Porcelaneous cement consists of a mixture of opal-A, opal-CT, and chert with montmorillonite and minor zeolite. This cement is an authigenic material precipitated in intergranular pore space. The origin of the opal is biogenic, with recrystalization of diatom frustules (opal-A) into opal-CT lepispheres and quartz crystals. Porcelaneous cement comprises 4-21% of the b lk volume of the rock. Seventy percent of the bulk volume of the cement is micropore space. As a result, 20-80% of the total porosity is microporosity in the porcelaneous cement.

Reservoir properties in these units are controlled by the relative amounts of biogenic and terrigenous sediments. In diatomite reservoirs, clean diatomaceous intervals are separated by the clay and silt-rich mudstones. In sandstone reservoirs, siliceous units replace true shales, having high water saturation and forming seals for oil-bearing sands. In addition, as siliceous material mixes into or precipitates within sand bodies, it reduces macroporosity, increases microporosity, and lowers permeability.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990