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Estimation of mineral compositions and pore volumes from wireline logging combinations (primarily gamma ray, neutron, density, and sonic) has been widely practiced for several years. The methodology is easily coded as a computer program whose output is a depth profile of mineralogic and/or porosity variation. It is possible to extend the rationale to the production of areal lithofacies maps by applying log analysis functions to interpolated grids produced by an automated contouring package. The result is a remotely sensed interpretation of geology based on physical measurements from a set of wells.
As a practical example, the composition of the Viola Limestone (Middle Ordovician) was mapped in a four-county area in southern Kansas through transformation of neutron, density, and sonic logs to estimations of calcite, dolomite, chert, and pore volume. Available cuttings and core information were used both to monitor the validity of the result and to provide meaning to observed variation. The map shows facies patterns which are readily related to depositional, diagenetic, and erosional trends of Viola crinoidal grainstone-packstone limestones and dolomitized, cherty mudstones and wackestones. The disposition of these units appears to be controlled by depositional-shelf geography modified by subsequent uplift and erosional beveling in both Late Devonian and Late Mississippian-Early P nnsylvanian. Weathered facies of a residual chert regolith and probable paleokarstic carbonates are also readily distinguished. The log analysis map shows a pronounced geographic grain whose directional features can be related directly to basement lineaments and their active role in tectonic uplift and erosion. Aside from its academic interest, the map is a useful exploration aid, because Viola oil fields in the area are located in both structural and combination traps.
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