Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
2003 West Texas Geological Society Fall Symposium, Midland, Texas, October 89, 2003
Deanna M. Combs, Eugene M. Kim, and Susan D. Hovorka
The Yates Formation in the Permian Basin, Texas contains an under-exploited gas resource. Little research or commercial effort has been directed at examining incremental gas production opportunities such as for the Yates Formation. Gas from the Yates Formation historically has been under-exploited in lieu of deeper occurring oil reservoirs and has been produced and sold at a discount due to its high nitrogen content (15-20 % nitrogen) and low pressures. Regional use of this gas as a fuel source for gas-fired turbines or mixing of the high nitrogen gas with other low nitrogen gases may increase its value and market. More detailed stratigraphic and reservoir characterization efforts are essential for designing efficient production strategies.
While the importance of cores to calibrate wireline log data is obvious, it is especially crucial in interpreting the lateral variations within the Yates Formation. Integrated log data and core studies of the Yates Formation from Ward-Estes, North Field, Ward County provide a basis for interpreting Yates facies and lithologies from wireline logs. Low gamma ray signatures in Ward-Estes, North Field wells correspond to intervals of thin dolomite and thick anhydrite. By contrast, cores in Kermit Field, Winkler County and Embar Field, Andrews County reveal that low gamma ray values represent thick intervals of halite, as well as the aforementioned dolomite and anhydrite. These data support a regional trend of increasingly more open marine facies in Yates sediments to the south and west in the Permian Basin.
Deposition within the Yates Formation is a function of cyclic marine flooding over shallow water exposure surfaces or tidal flats. Cycle bases are defined by marine sediments, such as dolomite, anhydrite and, halite, deposited during marine transgression. Lateral variations in lithology are probably due to local variations in marine circulation and topography. Cycle base marine units are (1) cryptalgally laminated, (2) brecciated due to solution collapse brecciation, (3) composed of well sorted, rounded rip-up clasts, or (4) karsted due to dissolution. Cycle tops are defined by thinly bedded, highly porous, fine-grained eolian sandstone to siltstone. Sandstone units are highly (1) haloturbated, (2) well bedded, or (3) massively bedded (rare). Although facies and thicknesses vary somewhat laterally, cycles are generally laterally continuous over large areas and relatively planar.
The contact between the Yates Formation and the underlying Seven Rivers Formation is gradational due to an overall regional decrease in accommodation as defined by the thinning of marine units and the thickening of eolian units. The upper contact of the Yates Formation and the overlying Tansill Formation is relatively sharp. This contact is interpreted to be the result of a large-scale marine transgression. The carbonate/anhydrite deposits that mark this flooding event are defined by an obvious gamma ray log response. Consequently, this contact is used as a regional correlation marker bed.