Geologic Description of Middle Devonian Chert Reservoir, Block 31 Field, Crane County, Texas

W. J. Ebanks Jr.

Block 31 field is a large anticlinal structure that produces from six pay zones. The first miscible gas-injection enhanced oil recovery project has been operated by ARCO since 1952 in the middle Devonian reservoir, which is the main producing interval in the field.

The middle Devonian unit was deposited as a siliceous, calcareous ooze in a basin-slope environment in the ancient Tobosa basin. Sedimentation buried earlier fault blocks, as the slope deposits aggraded and prograded mainly from the west and south. Subsequent diagenesis converted the ooze into calcareous chert.

Reservoir quality of the chert depends on degree of compaction and quartz cementation. Gray, dense, glassy chert at the base of the sequence is nonporous except for small, incipient fractures in the brittle matrix. Creamy white, evenly laminated, microporous, calcareous chert, which alternates with and overlies the dense chert in the lower half of the middle Devonian interval, has porosity of 10-30% and permeability as great as 10 md. The upper half of the reservoir interval consists mainly of light-gray and white, partially porous, heterogeneous, calcareous chert. This rock type has porosity of 5-20%, but permeability is usually less than 3 md. Patchy silica cement, discontinuous low-permeability laminae, and thin beds of dense limestone reduce its effectiveness as a reservoir. Euhed al dolomite and minor amounts of glauconite, pyrite, organic matter, and clay minerals occur in dark laminae and stylolitic seams in the calcareous chert. It has not been possible to interpret accurately lithology from well logs, because of the finely laminated and complexly varying mineralogy. However, porosity can be estimated from neutron-density logs, and zones of gas breakthrough in production wells can also be identified.

The environment of deposition of the middle Devonian sequence resulted in overall good continuity of gently inclined bedding, except where early faulting, submarine slumping, or other slope-failure processes interrupted the bedding. Diagenesis caused more subtle, lateral changes in porosity within beds. Structurally higher, southern portions of the field produce mostly from more homogeneous, porous, calcareous chert. Structurally lower northern parts of the field and flanks of the structure produce from more discontinuous, heterogeneous chert. This difference may account for differences in recovery efficiency and behavior of reservoir fluids in these different areas of the field.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.