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Stratigraphic Dipmeter Applications in Clastic to Carbonate Shoreline Transition: Core-Dipmeter Comparison

Anthony J. Lomando, Paul W. Britt

The James (Lower Cretaceous) shoreline complex cored in a southern Arkansas well consists of cyclic carbonate, clastic, and mixed lithologies. Arm-to-arm and button-to-button computations from the stratigraphic high-resolution dipmeter and core observations have been combined to illustrate the usefulness as well as limitations of the dipmeter in identifying and interpreting sedimentary and diagenetic features.

The core consists of a sandstone sequence with shoreface, beach, and island sedimentary features, overlain by a limestone marine-bar and channel-fill facies. The lower sandstone interval is an upward-fining sequence composed of skeletal sandstone, laminated to massive sandstone (with irregular porosity distribution), and carbonaceous sandstone, siltstone, and shale. The upper limestone unit is composed of laminated and cross-bedded ooskeletal grainstone overlain by an ooskeletal rudite containing gravel-size beachrock lithoclasts, coral, and bivalve shells.

The button-to-button dipmeter correlations were found to be most effective in discerning small-scale sedimentary features (< 4 cm), such as laminations and fine-scale cross-bedding due to variations in grain size, lithology, and porosity. Where bioturbation, compaction, or irregular porosity distribution have been overprinted on original bedding features, thin localized variations can be overcome by using the arm-to-arm computations. A larger cross section across the borehole combined with a larger correlation window reduced the effects of these very fine-scale irregularities. An expanded scale display of the 8 microresistivity curves was instrumental in qualifying the computed results. Using this display, horizontal porosity variations from diagenesis, bioturbation, and even stylo ites observed in the core could be recognized.

Computer processing often force-fits a planar solution to nonplanar features. By calibrating the dipmeter with a core, useful sedimentary features and their orientations can be identified. These relationships can then be extended to wells where dipmeter data is available, aiding in the development of area-wide depositional models.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.