Diagenesis and cement fabric of gas reservoirs in the Oligocene Vicksburg Formation, McAllen Ranch Field, Hidalgo County, Texas
- Univ. of Texas, Austin (USA)
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 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, and porosities range from 15 to 20%.
- OSTI ID:
- 5988794
- Report Number(s):
- CONF-9010204--
- Journal Information:
- AAPG Bulletin (American Association of Petroleum Geologists); (USA), Journal Name: AAPG Bulletin (American Association of Petroleum Geologists); (USA) Vol. 74:9; ISSN AABUD; ISSN 0149-1423
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
030200* -- Natural Gas-- Reserves
Geology
& Exploration
58 GEOSCIENCES
580000 -- Geosciences
ALKALINE EARTH METAL COMPOUNDS
ALLUVIAL DEPOSITS
CALCITE
CALCIUM CARBONATES
CALCIUM COMPOUNDS
CARBON COMPOUNDS
CARBONATE MINERALS
CARBONATES
CENOZOIC ERA
CHALCOGENIDES
CLAYS
DEPOSITION
DIAGENESIS
FEDERAL REGION VI
FELDSPARS
GEOLOGIC AGES
GEOLOGIC DEPOSITS
GEOLOGIC FORMATIONS
MINERAL RESOURCES
MINERALS
NATURAL GAS DEPOSITS
NORTH AMERICA
OXIDE MINERALS
OXIDES
OXYGEN COMPOUNDS
PERMEABILITY
POROSITY
QUARTZ
RESERVOIR ROCK
RESOURCES
ROCKS
SANDSTONES
SEDIMENTARY ROCKS
SILICON COMPOUNDS
SILICON OXIDES
SILTSTONES
TERTIARY PERIOD
TEXAS
USA