Structure, Facies, and Internal Properties of the Frio 'A' Reservoir, Hitchcock N. E. Field, Galveston County, Texas

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The N.E. Hitchcock field, which produces from the Frio 'A' or 9,100 ft sandstone, is defined by a northwest plunging anticline of moderate relief. It is truncated on its southeast flank by a major fault downthrown several hundred feet to the south. Several minor faults displace the original pay zone and may influence enhanced gas recovery efforts in the reservoir.

The Frio 'A' sandstone, which occurs at the top of the Frio Formation in the Chocolate Bayou area, forms part of a constructive delta lobe in the N.E. Hitchcock field. It consists of a stacked sequence of distributary-mouth-bar sandstones which grade into a thin delta destructional unit and are overlain by the transgressive shallow-marine Anahuac shale. Marine reworking of the Frio 'A' sandstone has resulted in its broad lateral extent and internal continuity although thin shale breaks vertically partition the reservoir. Much of the preserved excellent porosity (≥ 30%) and permeability (≥ 1,000 md, 0.99 μm²) in the Frio 'A' sandstone is due to its distributary-mouth-bar origin. The porosity and permeability were subsequently modified by diagenetic reactions.

The Frio 'A' aquifer extends some eight miles southwest of the N.E. Hitchcock field to the Alta Loma and Sarah White fields. It is confined on its northern and southern flanks by major growth faults. The continuity of the Frio 'A' sandstone has bearing on any plans to control water influx by drilling additional guard wells below the gas-water contact. However, reservoir modeling suggests that the faults are not sealing during co-production pressure drawdown.

Variable areas of indurated authigenic kaolinite zones developed in the Frio 'A' sandstone adjacent to thin shale units probably result from fluids emitted from the shales which have a consistent illite-smectite composition. Slight reductions in salinity during production at the Prets No. 1 well may be evidence for contemporaneous dewatering of shales.

Shale pyrolysis data indicate that the Anahuac and Frio shales are of too poor a quality and are too immature to have generated appreciable hydrocarbons. Furthermore, isotope data for the Prets No. 1 condensates imply a marine organic source for these fluids.

Thermal and hydrocarbon maturity data indicate that the Upper Frio was subjected to an extended period of hot, extremely saline, basinal-fluid flow. This fluid flow appears to have introduced the hydrocarbons and caused albitization of feldspars and formation of carbonate cements.


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