Unified Trap Model for Production Variation in Mississippian Reservoirs of the Cherokee Platform

Abstract

Unconventional drilling and completion techniques, initially established for developing gas shale fairways, are now being used to explore for and exploit the waste zones and transition zones of large fractured carbonate traps. The reservoir in these plays is regionally extensive, but varies widely in quality and structural position relative to free water. Improvement in oil prices and reduction in costs of drilling, completion and lifting technology required to produce and dispose of large volumes of associated water have led to commercial viability of even the poorest zones. The collective group of plays in the Mississippian section in Oklahoma forms a continuum of traps, waste zones, and transition zones in a large inter-connected reservoir system. Water-free oil accumulations in structural traps were discovered and exploited in the middle of the 20th century, using vertical drilling and open-hole completion. Horizontal drilling for these traps in the early part of this century yielded commercial production from thinner reservoirs at moderate oil pricing. Current conditions allow for vertical cased-hole production from down-dip edges and untapped compartments of the discovered conventional traps, as well as horizontal completions with multi-stage hydraulic fracturing in very low permeability reservoir facies of the dense Mississippian Lime. These wells rely on the presence of trapped oil within reservoir-seal couplets that can be gently liberated by horizontal completion, as long as through-going water-bearing vertical fractures can be avoided. With higher oil prices and the use of new 3D seismic, operators are also successfully exploring for complex conventional traps in more highly faulted areas around the Central Oklahoma Fault System and the Nemaha Ridge at the western edge of the Cherokee Platform. Case studies of each of these trap types align in time and space and provide a unified understanding of fluid distribution in the giant Mississippian carbonate system. A similar model describing the distribution of oil and water as a function of the interplay of pore geometry and structure may be at work in other fractured carbonate systems, such as the Permian and Niobrara.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015