ABSTRACT: Diagenesis and Cement Fabric of Gas Reservoirs in the Oligocene Vicksburg Formation, McAllen Ranch Field, Hidalgo County, Texas

R. P. Langford, F. L. Lynch

McAllen Ranch field produces natural gas from 12 deep, overpressured sandstone packages, each interpreted to be the deposit of a prograding shelf-edge delta. One hundred and sixty thin sections from 350 ft of core were petrographically described. The correlation of petrographic and sedimentologic features reveals a predictable pattern of sandstone cementation.

The sandstones are feldspathic litharenites containing subequal proportions of volcanic rock fragments (VRF), feldspar, and quartz grains. Grain size ranges from very fine to coarse sand. Porosity is mostly secondary, having formed through dissolution of VRF and feldspar grains. There are four major diagenetic facies (portions of core that can be grouped by the predominance of one diagenetic cement and similar appearance in hand specimen): (1) calcite cemented; (2) chlorite cemented, tight; (3) chlorite cemented, porous; and (4) quartz overgrowths, porous.

The calcite-cemented facies predominates in very fine grained sandstones and siltstones and encroaches into adjoining sandstones irrespective of grain size. Sparry calcite filled all available pores and replaced some feldspar. Core permeabilities are generally less than 0.01 md, and porosities range from 7 to 15%.

Authigenic clay (predominantly chlorite) generally cements sands intermediate in grain size between those cemented by calcite and those cemented by quartz. Two types of diagenetic clay fabric are interbedded, forming distinct alternating bands 0.1 in. to 3 ft thick. Gray, tightly chlorite-cemented bands are macroscopically and microscopically distinct from green, porous chlorite-cemented bands. In the tightly chlorite-cemented facies, permeabilities are less than 0.3 md, and porosities range from 8 to 16%. Small plates of chlorite fill interparticle pores, and secondary pores are rare. In the porous chlorite-cemented facies, dissolution of framework grains and chlorite cement increased porosity, and a second chlorite cement was precipitated. Core permeability ranges from 0.1 to 1 md, nd porosities range from 15 to 20%.

The quartz overgrowth facies occurs in 1 to 2 ft thick zones within the coarsest grained sandstones. Although these overgrowths constitute only 5% of the rock volume, they are the predominant cement. A relative abundance of intergranular pores results in permeabilities ranging from 1 to 20 md. Porosities range from 15 to 20%.

Three depositional facies are present in McAllen Ranch reservoirs: distal delta-front deposits that are characterized by turbidites, massive delta-front sandstones, and distributary-channel-fill sandstones. The diagenetic facies are localized into a distinct pattern within each depositional element. Distal delta-front turbidite strata are cemented by calcite at the base and top. Porous and tightly cemented chlorite bands occur in the middle of the turbidite upward-fining sequences, with the proportion of porous bands increasing toward the top. Within massive delta-front sandstones porous and tightly chlorite-cemented bands are commonly parallel to sedimentary structures. The calcite cement diagenetic facies is present in varying proportions. Distributary channel fills consist of thick r (10 to 20 ft), less well sorted, upward-fining sandstones. The basal 1 to 2 ft of the distributary channel fills, where underlain by shales, are tightly cemented with calcite. Porous, quartz-cemented diagenetic facies form 1 to 2 ft thick intervals in the coarsest grained sandstones. The rest of the coarse channel fill is dominated by porous, chlorite-cemented facies. The upper, finer grained portions of the channel fills are tightly cemented with chlorite and calcite. The predictable relationship between depositional facies and diagenesis allows extrapolation of the distribution of cements and resultant reservoir characteristics to uncored wells and intervals with the objective of maximizing recovery of natural gas.

AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990