Origins Of Water And Solutes In The Geopressured Zones Of The Northern Gulf Of Mexico Basin

Abstract

Abstract Detailed chemical analyses of 120 formation-water samples from 25 oil and gas fields in coastal Texas and Louisiana show that the salinity of water in the geopressured zones ranges from about 10,000 to 270,000 mg/L dissolved solids and may be higher or lower than the salinity of water in the normally pressured zones. Hydraulic fluid potentials and delta D pressured zones. Hydraulic fluid potentials and delta D and 6180 values indicate that the formation waters in the geopressured zones are most probably modified connate waters representing the original marine water of deposition. The chemistry of these waters, however, is markedly different from that of ocean water. The differences in composition resulted from physical and chemical interactions between water physical and chemical interactions between water and sediments. Introduction Waters with abnormally high pressures and temperatures are present at depths greater than about 3,000 m in Cenozoic and Mesozoic sedimentary rocks beneath an area of about 350,000 km in coastal Texas and Louisiana. The hydraulic pressure gradients in these geopressured zones (Fig. 1) are higher than hydrostatic [10.5 kPa/m (0.465 psi/ft)] and, with increasing depth, approach the lithostatic pressure gradient of 22.6 kPa/m (1.0 psi/ft). The pressure gradient of 22.6 kPa/m (1.0 psi/ft). The temperature gradients in the geopressured zones are variable but, for a given area, the gradient is higher than in the normally pressured zone. For example, in south Texas, the temperature gradients in the geopressured and normally pressured zones are 67 deg. C/km (3.7 deg. F/100 ft) and 34 deg. C/km (1.9 deg. F/100 ft), respectively. The depth at which geopressured zones are encountered varies from about 2,500 m in south Texas to more than 4,000 m in parts of coastal Louisiana (Fig. 1). This paper deals mainly with the origins of water and solutes in the geopressured zones, but is based on detailed chemical and stable isotope analyses of 120 formation-water samples from geopressured and normally pressured production zones ranging in depth from about 1,000 to about 5,600 m. The 25 oil and gas fields sampled are located in the McAllen-Pharr, Corpus Christi, and Houston-Galveston areas of coastal Texas, and the Lafayette area of coastal Louisiana. Oil and gas production is from sandstone reservoir rocks which, production is from sandstone reservoir rocks which, in Texas, are mainly in the Frio Clay of Oligocene(?) age and, in Louisiana, are of Miocene age. GEOLOGIC SETTING The fields sampled in this study are located within the Rio Grande, Houston, and Mississippi embayments on the northern margin of the Gulf of Mexico basin. This part of the basin is characterized by a very thick (up to 15,000 m) terrigenous sequence of shale, siltstone, and sandstone. The major sedimentary units are arranged in belts that parallel the present shoreline. Each younger unit parallel the present shoreline. Each younger unit overrides its predecessor, forming a new wedge of deposits that gradually increases in depth and thickness gulfward. Regional contemporaneous faults that are parallel to the present shoreline developed at the gulfward extremity of each wedges. Numerous salt domes, ridges, and massifs are important geological and geochemical features of the Gulf of Mexico basin. The "mother" salt for these structures is the Louann Salt of Jurassic age. Most of the samples collected from the Lafayette area, Louisiana, are from fields associated with known salt diapirs. No piercement salt structures were penetrated by drilling or are inferred from geophysical work in the fields sampled in the Houston-Galveston area. However, the fields in this area are located in an area of shallow salt diapirs (Fig. 2) and the Danbury field, located on a salt dome, is only about 15 km from the nearest wells sampled. There is no known or inferred salt structures in the vicinity of the fields sampled in the Corpus Christi and McAllen-Pharr areas of Texas (Fig. 2).