--> ABSTRACT: Depositional Environment of A. W. P. (Olmos) Field, McMullen County, Texas, by J. Greg Dennis; #91042 (2010)

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Depositional Environment of A. W. P. (Olmos) Field, McMullen County, Texas

J. Greg Dennis

A. W. P. (Olmos) field, located 3.5 mi east of Tilden, in McMullen County, Texas, has been extended 1,000 ft downdip or 6 mi south. Gas production has been established from the Olmos sand (upper Campanian) at depths as great as 11,200 ft. The step-out production has established a 2,000-ft productive column.

Biostratigraphic and lithologic data from cuttings in a downdip well, the Little Oil and Gas 1-A Huff, indicate the Olmos was deposited at an upper-bathyal depth of approximately 1,500 ft. The reservoir in the downdip wells consists of four distinct porous members that are correlative with four phases of progradation of a delta front represented by the thinner sequence of beds in the reservoir of the updip wells. Gross sand thicknesses range from 15 to 50 ft in the updip part of the field and from 50 to 100 ft in the downdip extension. The permeability of the reservoir decreases gradationally from 1 md updip to 0.01 md downdip, owing to an increase in clay content in the sediments deposited basinward.

Several wells drilled southwest and northeast of A. W. P. field, on strike with production, have tested the theory that the Olmos was deposited as a bar sand resulting from strong longshore currents reworking deltaic sediments. These wildcats have encountered Olmos sections extremely poor in sand, with less than 9% density-log porosity; 15 ft of 15% or greater density-log porosity is the "rule of thumb," that most field operators apply in decisions to set pipe. The presence of Olmos in A. W. P. field as a continuous unit from delta-front to submarine-ramp facies indicates that a delta-fed turbidite system existed in a calm Cretaceous sea, and has established that reservoir-quality Olmos sand in the area was deposited as an elongated body normal to the strike of the ancient shoreline, r northwest-southeast. Although the field sand can be correlated with a sand of apparently equivalent age present at 13,500 ft in a downdip Sligo test well, an economic limit is necessarily established at approximately 11,800 ft, where the pressure gradient and subsequent mud weights require that intermediate pipe be set to protect the shallow formations of the Queen City and Wilcox.

AAPG Search and Discovery Article #91042©1987 GCAGS and GC-SEPM Section Meeting, San Antonio, Texas, October 28-31, 1987.