Improving Recovery from Heterogeneous Clastic Reservoirs Using Borehole Image Sedimentology: Morrow Formation (Lower Pennsylvanian), Anadarko Basin, Oklahoma, U.S.A

Zarian, P.¹, R.D. Blumstein¹, S.A. Lomas¹, T.A. Vandeven² (1) Baker Atlas, Houston, TX (2) United Oil Corporation, Denver, CO

The Morrow Formation gas reservoirs on the northwestern shelf of the Anadarko Basin are an instructive example of fluvial to shallow marine clastic systems (incised valley fills, estuarine, deltaic to open marine) characterized by a very high degree of lateral and vertical variability. A major challenge to exploration and production of the Morrow sandstones is the subsurface mapping of the reservoir sand-bodies, whose apparent discontinuity and facies heterogeneity make them generally unamenable to conventional log-based well-to-well cor­relations.

Our multi-well integrated analysis demonstrates borehole image sedimentology as an approach to create geological models based on specific observational criteria from resistiv­ity borehole images. The wireline log responses of the Morrow sands lack distinctive char­acteristics for confident discrimination between different sand-body types. However, the high-resolution detail afforded by borehole imaging technology allows recognition of sedi­mentary features (e.g. bioturbation, scours, cross bedding, style of sand-on-sand contacts) and vertical stacking which can be used as criteria for facies-based differentiation of distinct depositional sub-systems (e.g. fluvial channel-fill, splay, marine bar, shoreface). In this way, the significance and potential continuity of sandstones encountered can be assessed deter­ministically, enabling prediction of fairways of productive sand facies.

The case-studies reservoirs discussed here were investigated in great detail using bore­hole image analysis to predict systematic down-dip facies distributions, paleotransport directions and sediment body geometries, which ultimately reduces geological uncertainty and thus facilitates decision-making for drilling and completion of future wells.