ABSTRACT

McAllen Ranch field produces natural gas from 12 deep, overpressured sandstone reservoirs, each interpreted to be the deposit of a prograding shelf-edge delta. 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. Four major diagenetic facies in cored reservoir rock can be grouped by the predominance of one diagenetic cement type and by 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. Core permeabilities are generally less than 0.01 md, and porosities range from 7% to 15%.

Authigenic chlorite generally cements sandstones intermediate in grain size between those cemented by calcite and those cemented by quartz. Two types of diagenetic chlorite fabric are interbedded, forming distinct alternating bands 0.1 in (0.25 mm) to 3 ft (1 m) thick. In the tightly chlorite-cemented facies, permeabilities are less than 0.3 md, and porosities range from 8% to 16%. In the porous chlorite-cemented facies, dissolution of framework grains and early yellow chlorite cement increased porosity, and a second coarsely crystalline chlorite cement was precipitated. Core permeability ranges from 0.1 to 1 md, and porosities range from 15% to 20%.

The quartz overgrowth facies occurs in 1- to 2-ft-thick (0.3- 0.6-m) 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: (1) mid-delta-front deposits that are characterized by thin upward-fining sequences; (2) massive upper delta-front sandstones; and (3) distributary channel-fill sandstones. The diagenetic facies are localized into a distinct pattern within each depositional element. Mid-delta-front upward-fining sequences 0.5 to 5 ft thick (0.2 to 1.6 m) are cemented by calcite at the base and top. Porous and tightly cemented chlorite bands occur in the middle of the 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. Distributary channel fills consist of thicker (10 to 20 ft), poorly sorted, upward-fining sandstones. The basal 1 to 2 ft (0.3 to 0.6 m) 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 (0.3- to 0.6-m-) 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. A predictable relationship between depositional facies and diagenetic facies allows extrapolation of the distribution of cements and the resultant reservoir characteristics to uncored wells and intervals with the objective of maximizing recovery of natural gas.