Depositional and Diagenetic Controls on Gas Production from Frio (Oligocene) Sands at Monte Christo Field, Hidalgo County, Texas

COGSWELL, THOMAS L., Mitchell Energy Corporation, The Woodlands, TX, and NOEL TYLER, Bureau of Economic Geology, University of Texas at Austin, Austin, TX

Depositional factors controlling the accumulation and production of hydrocarbons in Monte Christo field, central Hidalgo County, are related to lateral and vertical facies changes as well as syndepositional growth faulting that has compartmentalized the pay sands into several discrete, fault-bounded reservoirs. Rollover into these growth faults, coupled with greater sand thickness and quality at these preferred points of delta-front sand deposition, result in a combination of structural and stratigraphic entrapment of gas.

Frio reservoir sands were deposited in the Norias delta system of the Rio Grande embayment. Lower-middle Frio depositional facies at Monte Christo field consist of stacked, upward-coarsening, progradational packages. In each, a basal abandonment facies is overlain by prodelta muds. Overlying these are distal delta-front sands and silts (lower shoreface) that display classic turbidite sequences. Distributary mouth bar (upper shoreface) sands comprise the best reservoir facies and cap each package. During Oligocene times, the area was characterized by rapid rates of sedimentation and contemporaneous down-to-basin faulting. The large Mission fault bounds the field to the west and parallels depositional strike. The F-4 through F-6 reservoir intervals and similar related sands do not corre ate to the west across this fault, and these cyclic packages are the direct result of episodic movement along the Mission fault. Within the study area, three smaller splinter faults bifurcate to the east from the Mission fault and structurally compartmentalize the Frio reservoirs. These splinter faults are the result of greater displacement along the Mission fault toward the main depocenter to the south, and sand thicknesses increase in that direction.

Diagenetic influences on reservoir quality involve extensive cementation and secondary porosity generation. Reservoir sands are fine- and very fine-grained feldspathic litharenites and lithic arkoses dominated by plagioclase, volcanic rock fragments, and volcanic quartz derived from Tertiary volcanics of the Trans-Pecos region to the west. The breakdown of unstable volcanic rock fragments results in significant chlorite and illite grain coating cements that commonly block pore throats and in some instances pervasively cement entire laminae. Extensive calcite and ferroan carbonate cementation further occludes porosity. Minor quartz, potassium feldspar, zeolite, and iron and titanium oxide authigenesis is present as well. These factors result in relatively low average porosities (5 to 1 %) and very low permeabilities.

The generation of secondary porosity by dissolution of feldspars and perhaps some carbonate material greatly enhances the pore volume. This secondary porosity was generated late in the diagenetic history and was likely associated with CO2 and organic acids migration following expulsion from source material in the early phases of the hydrocarbon maturation. Point counts reveal that a minimum of 35% of the existing porosity is of secondary origin. Even with significant porosity enhancement in some intervals, hydraulic fracturing is required to achieve an economic rate of production in most cases.

AAPG Search and Discovery Article #91006 © 1991 GCAGS and GC-SEPM Meeting, Houston, Texas, October 16-18, 1991 (2009)