Recovery from Mature Oil Fields Producing from Upper Jurassic Carbonate
Mancini, Ernest A.1, Thomas A. Blasingame2, Rosalind Archer3, Brian J. Panetta1, Juan Carlos Llinás1, Bennett L. Bearden1 (1) University of Alabama, Tuscaloosa, AL (2) Texas A&M University, College Station, TX (3) University of Auckland, Auckland, New Zealand
Reservoir characterization, modeling and simulation were undertaken to improve production from mature oil fields producing from carbonate reservoirs that are associated with petroleum traps resulting from salt tectonics. These fields produce oil from Upper Jurassic (Oxfordian) carbonate shoal reservoirs. These reservoirs occur in vertically stacked, heterogeneous depositional and porosity cycles. The cycles consist of lime mudstone/wackestone at the base and ooid grainstone/packstone at the top. The lime mudstone and wackestone lithofacies have been interpreted as restricted bay and lagoon sediments, and the grainstone and packstone lithofacies have been described as shoreface and shoal deposits. Although the primary control on reservoir architecture is the depositional fabric, diagenesis is a significant factor in modifying reservoir quality. Porosity has been enhanced through dissolution and dolomitization. Porosity is chiefly interparticle, solution-enlarged interparticle, grain moldic, intercrystalline dolomite and vuggy pores. Dolostone pore systems and flow units have the highest reservoir potential. Petroleum trapping mechanisms include fault traps (footwall uplifts with closure to the south against major west-east trending normal faults), footwall uplift traps associated with minor south-north trending normal faults, and saltcored anticlines with four-way dip closure. Potential barriers to flow are present as a result of petrophysical differences among and within the cycles as well as the presence of normal faulting. Reservoir performance analysis and simulation indicate that both unitized and nonunitized fields have oil remaining to be recovered. A field-scale reservoir management strategy that includes the drilling of infill wells in structurally high areas and perforating existing wells in stratigraphically higher porosity zones is recommended for sustaining production from these fields.