Abstract

Leonardian carbonate reservoirs in the Permian Basin have produced 2.8 billion stock-tank barrels (BSTB) of the estimated 13.6 BSTB original oil in place (OOIP), for a current recovery efficiency of 20 percent. Of the unrecovered oil, 6 BSTB is residual, whereas 4.8 BSTB is remaining mobile oil. This 4.8 BSTB mobile oil resource is a significant target for strategic infill drilling and waterflood optimization. Characterization of 130 of the largest Leonardian carbonate reservoirs in the Permian Basin permitted the comprehensive analysis of geologic and engineering attributes and the classification of these reservoirs into a play framework. These reservoirs are subdivided into four geologic plays: Northern Shelf Abo Reef Trend, Leonardian Platform Carbonate, Northern Shelf Permian Carbonate, and Eastern Shelf Permian Carbonate.

Geologic and engineering properties were analyzed within this play framework. The Northern Shelf Abo Reef Trend is the deepest play, producing primarily from reef limestones. This play has produced 0.5 BSTB of oil since the first reservoir was discovered within it in 1951. Reservoirs in this play have had an average deliverability of 1400 MSTB per year and are primarily under 40-acre secondary recovery projects. The Leonardian Platform Carbonate play is at medium depth, producing from all portions of the Leonardian Series. These reservoirs are a mixture of dolostone, limestone, and sandstone and produce mainly from simple anticlines under solution-gas drive. This play has produced 1.4 BSTB since its first reservoir was discovered in 1941, accounting for 18 percent of its OOIP. Reservoirs have had an average deliverability of 500 MSTB per year and are primarily under 20- to 40-acre waterflood projects. The Northern Shelf Permian Carbonate play is the second deepest play, the Tubb formation being distinctly unproductive. These reservoirs are almost exclusively dolomitized, producing from a combination of simple anticlines and stratigraphic traps under solution-gas drive. They have produced 0.7 BSTB since the first reservoir in the play was discovered in 1940, accounting for 32 percent of the play’s OOIP. The average reservoir deliverability has been 700 MSTB per year. Reservoirs are currently under 40-acre waterflood. The Eastern Shelf Permian Carbonate play is the shallowest, producing from the Glorieta and Clear Fork Formations but not from the Tubb. Reservoirs in this play produce from simple anticlines composed of dolostone and limestone and have both solution-gas and water-drive mechanisms. These reservoirs have produced 0.3 BSTB since the first reservoir in the play was discovered in 1921, accounting for 24 percent of the play’s OOIP. The Eastern Shelf Permian Carbonate reservoirs are the most highly developed, with nearly all under 20-acre waterflood.

Of these four geologic plays the Leonardian Platform Carbonate play contains the greatest reservoir complexity and the lowest current recovery efficiency. The wide range of lithologies and producing intervals combined with the lowest ratio of net-pay to gross-pay thickness makes this set of reservoirs the most geologically heterogeneous. An average of 23 years of waterflooding and moderate well spacing and reservoir compartmentalization produced by geologic heterogeneity indicate potential increased oil recovery from both strategic infill drilling and waterflood profile modification. Strategic recovery practices within this play, which take into account all the geologic and engineering factors, incorporate a target of 2.7 BSTB of remaining mobile oil.