ABSTRACT

Significant amounts of hydrocarbons have been produced from Oligocene Vicksburg sandstones in Lyda and North Rincon fields in Starr County, Texas at depths ranging from 5,000 to 9,500 feet. Detailed examination of cores has shown that Vicksburg sands in the lowermost intervals were deposited in the upper delta plain as lacustrine deltaic deposits. Although no whole core was available in the mid to upper sand intervals, they appear to have been deposited in a shallow marine deltaic complex (based on electrical logs, paleontological data, and wireline cores). Diapirism of the Eocone Jackson shale, along with associated growth faulting, generated highly faulted, rollover elongate anticlines. The bulk of production in the two fields is from these faulted highs and from stratigraphic pinchouts.

The Vicksburg sands were studied in detail with the petrographic microscope, x-ray diffraction, and the scanning electron microscope to determine the parageneisis of the sands. The sandstones varied in lithology from feldspathic litharenites to lithic arkoses. An Eocene volcanic source in the Trans-Pecos region was responsible for the large amount of feldspars (mainly albite) and volcanic rock fragments found. The abundance of unstable mineral components results in a complex diagenetic sequence.

Most of the effective porosity found in the Vicksburg sands is of a secondary nature and was created by the dissolution of feldspars, volcanic rock fragments, and calcite cement. Permeability has been significantly reduced in some porous sands by the precipitation of calcite cement, iron-chlorite (chamosite) and to a lesser extent, kaolinite and illite/smectite mixed layered clays. Chlorite was the most common authigenic clay and was found in significant amounts throughout the entire Vicksburg section. The best reservoirs in the Vicksburg sands were those with the largest average grain size and the greatest amount of feldspars and volcanic rock fragments prior to diagenesis.