Identification and Assessment of Remaining Oil Resources in the Frio Fluvial-Deltaic Sandstone Play, South Texas

The Frio Fluvial-Deltaic Sandstone (Vicksburg Fault Zone) play of South Texas has produced nearly 1 billion barrels (Bbbl) of oil since field development began in the 1940's. More than half of the reservoirs in this depositionally complex play have been abandoned, although large volumes of oil remain. A sizeable resource may remain unproduced unless methods integrating geological and engineering reservoir characterization are developed to identify the location of unrecovered mobile oil, estimated at more than 1 Bbbl, that remains in undrained reservoir zones in fields within this mature play.To estimate the volume of oil resources remaining in this prolific play and to assess the potential of reservoirs for containing zones that have additional unproduced oil, geologic and engineering characteristics of reservoirs in fields throughout the play were evaluated. Sandstone reservoirs within the play represent depositional facies that include aggradational fluvial-channel sandstones, mixed aggradational-progradational fluvial- and distributary-channel sandstones, and progradational delta-front sandstones. The differing stratigraphic context of these types of fluvial-deltaic reservoirs within the play results in differences in engineering attributes and internal heterogeneity that may significantly affect reservoir quality and hydrocarbon recovery efficiency, thereby affecting the potential for additional recovery.Reservoir engineering attribute data were statistically analyzed from oil and gas fields throughout the geographic play area. General reservoir attributes used to characterize Frio fluvial-deltaic sandstone reservoirs included porosity, initial water saturation, residual oil saturation, net pay, reservoir area, and fluid characteristics. Statistical analysis of variance demonstrated no difference between oil and gas reservoir attributes, supporting the interpretation that oil and gas reservoirs are subsets of a larger genetically similar population. Probability functions that best describe attribute frequency distributions were determined for use in risk-adjusting resource calculations. Different probability functions were found to be most applicable for the various petrophysical reservoir attributes. Statistical tests suggest that the Weibull function most accurately represents the frequency distribution of reservoir volumes, including original oil in place (OOIP), original mobile oil in place, and residual oil in place. This finding is in contrast to convention that generally assumes a lognormal distribution. Use of a Weibull distribution to simulate probable reservoir volumes results in lower values for the oil-in-place resource estimate than those calculated by using average reservoir parameters.The Frio Fluvial-Deltaic Sandstone play contains large volumes of remaininoil. The volumetric probability distribution between 5- and 95-percent probability for OOIP ranges from 3.8 to 5.6 billion stock-tank barrels (BSTB), original mobile oil in place ranges from 2.5 to 3.6 BSTB, and residual oil ranges from 1.5 to 2.3 BSTB. The remaining mobile oil, at the 95-percent probability level, is at least 1.2 BSTB. Additionally, the untapped oil resource (sandstone bodies not connected to a well bore) may be 10 percent of the OOIP, or 380 million stock-tank barrels (MMSTB).Reservoir characterization coupled with proper reservoir management techniques can be applied to recover addition mobile oil in this play as well as in other analogous reservoirs.