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Abstract: Depositional and Diagenetic Fabric of
Gas Reservoirs in the Oligocene Vicksburg
Formation, McAllen Ranch Field,
Hidalgo County, Texas
McAllen Ranch gas field in Hidalgo County has been studied as part of an effort to understand reservoir heterogeneity within mature natural gas fields as a means to detect uncontacted and bypassed gas pools. McAllen Ranch produces from overpressured sandstone reservoirs of the Oligocene Vicksburg Formation. Structurally, the field consists of a faulted, shingled set of southeastward tapering structural wedges. Deposition was contemporaneous with deformation, resulting in a complex interplay between sedimentation and tectonism.
The sandstones contain 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. Three major diagenetic facies in cored reservoir rock can be grouped by the predominance of one diagenetic cement type: (1) calcite-cemented, (2) chlorite-cemented, and (3) quartz overgrowth-cemented. Permeability variation of more than two orders of magnitude within the reservoir sandstones corresponds to different diagenetic facies.
The calcite-cemented facies predominates in very fine-grained sandstones and siltstones and encroaches into adjoining sandstones irrespective of grain size. Authigenic chlorite fabrics are interbedded, forming distinct alternating bands 0.1-inch to3-feet thick. The quartz overgrowth facies occurs in 1- to 2-ft-thick zones within the coarsest grained sandstones.
Three depositional facies in McAllen Ranch contain reservoir quality rock: (1) mid-delta-front deposits; (2) massive upper delta-front sandstones; and (3) distributary channel-fill sandstones. Mid-delta-front deposits coarsen upward overall and consist predominately of thin, 3-inch- to 5-foot-thick upward-fining sequences that have scoured bases. These upward-fining sequences are cemented by calcite at the base and top. Proximal delta-front deposits are upward-coarsening intervals interpreted to be either delta mouth-bar crest or similar upper-shoreface deposits. Poorly defined laminae are the most common sedimentary structure. Porous and tightly chlorite- and calcite-cemented bands commonly parallel sedimentary structures. Distributary channel fills consist of thicker (10 to 20 foot), poorly sorted, upward-fining sandstones. The basal 1 to 2 feet of distributary channel fills, where underlain by shales, are tightly cemented with calcite. Porous, quartz-cemented diagenetic facies form 1- to 2-foot-thick bands with the coarsest channel fill.
Differences in the character of the microresistivity curve of the high-resolution dipmeter log correlate with differences in cementation and with different depositional facies within the cores. Examination of microresistivity logs and high-resolution density logs allows accurate prediction of diagenetic facies and resulting reservoir properties.
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