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ABSTRACT: Interpretation of Small-Scale Geologic Features and Depositional Facies Analysis Using Borehole Images

Laura Stager Foulk, Robert A. Young

Critical geologic features indicating the depositional facies of a sedimentary sequence are often small and, therefore, cannot be directly inferred from conventional wireline logs. However, recognition of smallscale geologic features, down to 1.25 cm (0.5 in.), is now routinely accomplished using high-resolution resistivity images.

Facies analysis studies using images employ techniques similar to facies analysis using core. Facies analysis involves three steps: (1) recognition of internal sedimentary structures such as thin beds, crossbeds, convoluted bedding, fractures, bioturbation, shell fragments, concretions, and vugular porosity; (2) identification of bedding plane geometry and bed stacking patterns; and (3) fitting these observations into a facies model appropriate to the regional facies interpretation.

Deep-water turbidite facies are recognized on images as thin regular laminations, usually in cyclic depositional units. Bioturbated facies, characterized on images by shell fragments, vertical burrows, and lateral disruptions of interbedded sand/shale laminations, indicate low-energy shallow-water shelf environments. Slumping, convoluted bedding and other soft sediment deformation are seen on images of deep-water slope or fan channel depositional units. Thin sands, interbedded with shale, thicken and coarsen upward in images from distal delta environments. All these environments have been identified through image studies in the Gulf of Mexico region.

Results from image facies analysis augment core facies analysis, bringing core on-depth, identifying missing core sections, and extending reservoir evaluation to noncored zones.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990