ABSTRACT

Monte Christo Field is located in central Hidalgo County on the Gulf Coastal Plain of south Texas. Depositional and structural factors controlling the accumulation and production of hydrocarbons are related to lateral and vertical facies changes as well as syndepositional growth faulting. These factors combine to compartmentalize the pay sandstones into discrete, isolated reservoirs. Rollover into these growth faults, coupled with greater sandstone thickness and quality at preferred points of delta-front sand deposition, result in a combination of structural and stratigraphic entrapment of gas.

Frio reservoir sandstones consist of stacked, upward-coarsening, progradational packages. In each is found a vertical succession of basal abandonment facies, offshore mudstones, distal-delta front sands and silts (lower shoreface), and distributary mouth bar (upper shoreface) sandstones. The upper shoreface sandstones comprise the best reservoir facies. Each facies contains trace fossils indicative of the paleo-environment.

Cyclic progradational packages at Monte Christo Field are the direct result of episodic movement along the Mission fault, a large down-to-basin growth fault which bounds the field to the west and parallels depositional strike. Within the study area, three smaller splinter faults bifurcate to the east from the Mission fault and compartmentalize the Frio reservoirs. Diagenetic influences on reservoir quality involve extensive cementation and secondary porosity generation. Reservoir sandstones are fine- and very fine-grained feldspathic litharenites and lithic arkoses. The breakdown of unstable volcanic rock fragments results in significant chlorite and illite grain-coating cements which commonly block pore throats and greatly reduce permeability. Calcite and ferroan carbonate cementation occludes porosity significantly. Minor quartz, potassium feldspar, zeolite and titanium oxide authigenesis are also present. These factors result in relatively low porosities of 10 to 15% and very low permeabilities.

The generation of secondary porosity by dissolution of feldspars and carbonate material enhances the diminished pore volume. Point counts reveal that a minimum of 30% of the existing porosity is of secondary origin.

Even with significant porosity enhancement in some intervals, artificial fracture stimulation is required to achieve an economic rate of production. This procedure also taps into the substantial quantities of gas trapped in the lowest permeability portions of the reservoir facies.