Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
2003 STGS/GCSSEPM One-day Research Conference, San Antonio, Texas, April 11, 2003
Description of a Lower Cretaceous Glen Rose "Patch Reef" in the Prime Operating Company #1-84 La Paloma Well from the Chittim West Field, Maverick County, Texas
Robert G. Loucks and Charles Kerans
Several important carbonate plays have developed in the Lower Cretaceous Glen Rose section in Maverick County in South Texas. As reported by Sams (2002) and O'Brien (2002), the two plays are (1) patch reefs and (2) nonreef-associated vug-enhanced fractured carbonates. The Prime Operating Company core recovered 85 feet from a 120-foot section. The patch reef section is greater than 70 feet thick; however, 35 feet of the lower section was not recovered.
Fifty-two feet of stratigraphic section below the biohermal facies consists of several rock types deposited in deep to moderately deep water. Unit 1 is a burrowed, wispy-bedded Orbitolina lime wackestone to a mud-rich lime packstone. Unit 2 is a burrowed, wispy-bedded, argillaceous lime mudstone containing ostracodes, calcispheres, and echinoid and mollusk fragments. A collapsed burrowed zone marks the top of the unit. Unit 3 and is a lime packstone containing algal(?)-coated grains. Unit 4 grades upward from a lime wackestone to a mud-rich lime packstone. The top of the unit has Orbitolina forams and the first occurrence of requienid fragments.
Unit 5 is an intensely burrowed, mud-rich, slightly dolomitic lime packstone that increases in grain content upward. Requienid fragments (a few whole requienids) are present throughout the section. The section appears to grade into the biohermal facies above. The lower facies of the biohermal facies, Unit 6, consists of a mud-rich, slightly dolomitic lime packstone containing whole abundant requienids, along with a few caprinids, stromatoporoids, and corals. This unit contains the missing core section; however, the porosity curves on the wireline logs suggest that the missing section is probably similar to the rest of Unit 6.
The lime boundstone (bafflestone and bindstone) section, Unit 7, is 18 feet thick. It consists of requienids, stromatoporoids, corals, sponges(?), Chondrodonta, rare caprinids, binding stromatoporoids and Lithocodium, and echinoid and mollusk fragments. Above the boundstone facies is at least 5 feet of coarse-grained lime rudstone (Unit 8) that has the same components as the boundstone.
The bioherms are found in the transgressive systems tract of the longer term Glen Rose composite sequence but in the highstand of the lower Glen Rose high-frequency sequence, below the Corbula(?) beds. In this setting, we anticipate a string of subparallel isolated buildups rather than a more continuous barrier associated with a late highstand prograding system. Depositional relief on the buildups was as much as 70 feet based on the vertical facies sequence.
The section having moderate reservoir quality includes Units 5 through 8 of the patch reef and associated facies. Pore types are dominated by moldic pores and micropores and some interparticle and intraparticle pores. Average porosity is 9.2% (range is 2.1% to 15.1%), and average permeability is 2.9 md (range is 0.02 to 36.2 md). Units 1 through 4 are deeper water, mud-rich units, and the reservoir quality is much lower than that of the patch reef facies. In these units average porosity is 1.8% (range is 0.8% to 4.1%), and average permeability is 0.5 md (range is 0.01 to 4.5 md). Some of the permeability in these tighter facies may be a result of fracturing of the core plug, as indicated by a near-vertical porosity versus permeability relationship.
The well was perforated in the boundstone facies between 5,266 and 5,276 feet and had an initial production test (IPF) of 2.1 MMcf of gas per day with a bottom-hole shut-in pressure of 2,346 psi. It was treated with 1,000 gallons of 15% HCl. This well has produced 1.3 billion cubic feet of gas, 8,842 barrels of oil, and no water since production started in 1993.
Patch reefs, such as those seen in the Price Energy #1-84 well, should be common in the lower Glen Rose section in South Texas. The best exploration approach for these reservoirs is seismic analysis. Both subtle seismic buildups and amplitude anomalies may indicate patch reefs.
Sams, R. H., 2002, Carbonates regain the South Texas spotlight: South Texas Geological Society Bulletin, October, p. 1127.