ABSTRACT

A study was undertaken to determine if the various facies represented in the carbonate Sligo formation (Lower Cretaceous) could be defined by well log analysis in a six county area along the platform margin of this formation in south Texas. Complete suites of well logs penetrating the Sligo from some 36 wells were used within an area where excellent geologic data based upon examination of over 900t of core from 50 wells had defined the facies and environments of deposition. The carbonate system consists of a complex series of vertically sequenced supratidal oolite-shoals, grading to pack stones, both of which contained porosity and permeability. These, in turn, grade into non-permeable mud-supported wacke stones and mudstones in low energy environments, both down dip in the lower shelf environments and up dip in the intertidal regions. Various well log crossplots indicate that each of the major facies units can be readily distinguished by use of well logs where porosities are above 5%, with common slopes for each facies identified within this region. Thus, logs can be used generically to both identify Sligo carbonate facies and to help reconstruct the geologic environment where core data are incomplete. This type of analysis can be of great value in determination of lateral extent and continuity of individual reservoirs within carbonate systems. Additional study is needed to determine if these relationships hold in other areas, and in other geologic formations.

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