Pressures, Hydrocarbon Accumulation, and Salinities Chocolate Bayou Field, Brazoria County, Texas

Abstract

The hydrocarbon accumulation at Chocolate Bayou field appears to be intimately related to the fluid environments there. An understanding of the processes at work in this field can be used in planning the development of active fields where comparable fluid data are available. Introduction The Chocolate Bayou field is located in east-central Brazoria County, about 25 miles south of Houston, Tex. Production in the field is from Oligocene Frio sands ranging in depth from 8,600 to 12,200 ft. About half the approximately 2Tcf of gas reserves in the field is found in reservoirs with pressures substantially above hydrostatic. Data collected since the discovery of this field in 1939 permit an evaluation of the effects of abnormally high pressures on the migration and entrapment of hydrocarbons and the salinity of formation waters in the field. An understanding of the relationship between fluid properties and hydrocarbon entrapment in this field may be applicable to the development of other Gulf Coast fields. Geology The Chocolate Bayou field produces from a faulted anticline in the downdip Frio trend. A large growth fault divides the field into two producing areas. Fig. 1 shows the producing area of the field and the approximate location of the fault separating the two principal producing segments. The downthrown, eastern fault producing segments. The downthrown, eastern fault block produces from a closed anticlinal structure; the upthrown, western fault block produces from a closure against the regional fault crossing the field. A number of minor faults are present, but have little effect on the pattern of hydrocarbon accumulation in the field. The two major fault blocks are referred to as East Chocolate Bayou and West Chocolate Bayou in this paper. Structural maps of the field are given by Allen and Allen and Myers. The producing Frio section is an alternating sand and shale sequence overlying a thick section of Oligocene and older shales. Above the Frio is the Anahuac shale wedge, which is about 2,200 ft thick in the field. About 6,500 ft of undifferentiated Miocene and younger massive sands interbedded with comparatively thin shales extends from the top of the Anahuac to the surface. Radical changes in the lithology and thickness of the Frio section across the growth fault bisecting the field indicate that movement along this fault was contemporaneous with deposition of the Frio field pays. Movement continued intermittently along the fault during the deposition of the Anahuac shales and the lowermost Miocene sands. The increased Frio sand deposition in the area of the present structural high suggests that movement of the producing structure had already begun by Frio time. Uplift continued well into Miocene, where the feature is expressed as a faulted nose at depths as shallow as 6,000 ft. This long history of structural uplift has resulted in an increase of structural relief with depth and an abundance of minor faults associated with this structural deformation. JPT P. 411