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

KOPASKA-MERKEL, DAVID C., and STEVEN D. MANN, Geological Survey of Alabama, Tuscaloosa AL

Quantitative 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 (H) is {[(0.25 s porosity) + (mean ln permeability) + (1.5 s ln permeability)]/3}. Megascopic heterogeneity (MH) is [(number of reservoir intervals) + (number of high-permeability reservoir intervals) + (s of number of reservoir intervals)] where reservoir rock is defined as exhibiting permeability values greater than/equal to 0.1 md and high-permeability reservoir rock exhibits permeability values greater than/equal to 1.0 md. Both megascopic heterogeneity and microscopic heterogeneity are determined from core data. The Dykstra-Parsons coefficient (DP) is a measure of microscopic heterogeneity that is partially independent of microscopic heterogeneity (r{2} = 0.428). All three of these parameters are primarily measures of vertical heterogeneity, although averaging of wells within a field incorporates lateral heterogeneity in microscopic- and megascopic-heterogeneity, and DP.

Microscopic heterogeneity and megascopic heterogeneity are distributed in contrasting but related patterns. Microscopic heterogeneity generally decreases from northwest to southeast; highest values are 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. H values are high in the Moldic Pore Facies and low in the Intercrystalline Pore Facies. The distribution of megascopic heterogeneity is roughly opposite to that of microscopic heterogeneity. Megascopic heterogeneity values are high on the north flank of the Wiggins arch, on the onecuh 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 microscopic heterogeneity and megascopic heterogeneity. Also, the low-relief north-south trending salt-cored anticline in western Washington County is characterized by relatively high values of megascopic heterogeneity. Reservoirs belonging to the Moldic Pore Facies tend to be homogeneous with respect to megascopic heterogeneity, whereas reservoirs assigned to the Intercrystalline Pore Facies are characterized by relatively high values of megascopic heterogeneity.

Megascopic Heterogeneity and microscopic heterogeneity 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 micro- and megascopic heterogeneity 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 reg onal heterogeneity studies than is megascopic heterogeneity or microscopic heterogeneity.

AAPG Search and Discovery Article #91014©1992 AAPG GCAGS and GC-SEPM Meeting, Jackson, Mississippi, October 21-23, 1992 (2009)