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The AAPG/Datapages Combined Publications Database

GCAGS Transactions


Gulf Coast Association of Geological Societies Transactions
Vol. 42 (1992), Pages 189-212

Regional Variation in Microscopic and Megascopic Reservoir Heterogeneity in the Smackover Formation, Southwest Alabama

David C. Kopaska-Merkel, Steven D. Mann


Quantitative (rank) measures of microscopic and megascopic reservoir heterogeneity are used to characterize the distribution of reservoir heterogeneity in Smackover hydrocarbon fields and wildcat wells in southwest Alabama. Microscopic reservoir heterogeneity (µH) is {[(0.25^sgr^phgr) + (meanlnK) + (1.5^sgrlnK)]/3}. Megascopic Heterogeneity (MH) is [(# of reservoir intervals) + (# of high-K reservoir intervals) + (^sgr of # of reservoir intervals)] where reservoir rock is defined as exhibiting permeability values >= 0.1 md and high-permeability reservoir rock exhibits permeability values >= 1.0 md. Both MH and µH are determined from core data. The Dykstra-Parsons coefficient (DP) is a measure of microscopic heterogeneity that is partially independent of µH(r2 = 0.428). All three of these parameters are primarily measures of vertical heterogeneity, although averaging of wells within a field incorporates lateral heterogeneity in µH and DP.

µH and MH are distributed in contrasting but related patterns. µH generally decreases from northwest to southeast, with the highest values found in the vicinity of the Choctaw Ridge complex north of the Mississippi Interior Salt Basin (MISB). Moderately high values typify the Manila Embayment and the Conecuh Ridge complex to the south, whereas lower values are found in the MISB, on the north flank of the Wiggins Arch, and in the Conecuh Embayment. µH values are high in the Moldic Pore Facies and low in the Intercrystalline Pore Facies. The distribution of MH is roughly opposite to that of µH. MH values are high on the north flank of the Wiggins Arch, on the Conecuh Ridge complex and in the Conecuh Embayment: MH values are low near the Choctaw Ridge complex. The Conecuh Ridge is unique because it is characterized by high values of both µH and MH. Also, the low-relief north-south trending salt-cored anticline in western Washington County is characterized by relatively high values of MH. Reservoirs belonging to the Moldic Pore Facies tend to be homogeneous with respect to MH, whereas reservoirs assigned to the Intercrystalline Pore Facies are characterized by relatively high values of MH.

MH and µH vary congruently with pore-system characteristics (controlled by depositional patterns, dissolution, and dolomitization) and regional structural and paleogeographic trends. This suggests that reservoir heterogeneity characteristics are controlled by structural and paleogeographic setting, by depositional fabric, and by diagenesis. However, because contours of µH and MH are approximately normal to structure contours but parallel to Smackover thickness contours, it appears that depositional setting (or paleogeography) influenced reservoir heterogeneity more than did structural evolution. The distribution of DP values is not related to pore-facies distribution. Thus we conclude that DP is less useful for regional heterogeneity studies than is MH or µH.

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