ABSTRACT: Understanding Flow Units and Depositional Processes before Secondary Cycling in Lower Stevens Turbidites, California

JORDAN, D. W., ARCO International Oil and Gas Company, Plano, TX

The upper Miocene, Lower Stevens Sandstone in the Paloma field (San Joaquin Valley, California), a retrograde condensate and gas reservoir, was deposited as a series of shingled "wedges" in a deep marine basin. Conventional core material and logs were used to provide a reservoir description for determining recovery efficiency for secondary cycling.

A typical sequence in the lower one-third of the 100-250 m thick Lower Stevens interval consists of a graded and upward-fining sedimentation unit comprising four subunits: (A) massive-appearing to faintly laminated, moderately sorted, medium- to coarse-grained sandstone with numerous internal contacts that was deposited by high-density currents (0.3-1.3 m thick), overlain by low density deposits consisting of (B) very fine to fine-grained sandstone with local dewatering features (30 cm thick), changing upsection to (C) "slurried" carbonaceous silty sandstone of silty claystone with carbonaceous siltstone fragments and rare burrows (6-18 cm thick), capped by (D) rippled to laminated silty claystone and claystone (3-9 cm thick). Higher permeabilities (3-8 md) are confined to subunit A. ertical transmissivity within a single well is poor because of low permeability in subunits B-D. Multiwell vertical transmissivity is good (because low permeability subunits are rarely laterally extensive) but tortuous. Fine subunits act as reservoir baffles. Horizontal continuity is good to excellent but "stair-stepped" as a result of lateral facies changes in the subunits.

Many sandstones in the upper two-thirds of the Lower Stevens are an amalgamation of high-density A subunits. The absence of B-D subunits was caused by erosion or transportation of the fine units basinward. The result is a thick, single, permeable flow unit having excellent vertical and lateral continuity. Cycling efforts should be initially concentrated in this zone because much of the condensate will be sufficiently contacted and there is a high chance for excellent throughput. In addition, these sandstones were not perforated in all cases and are candidates for recompletion.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)