--> The Mancos Shale in the southeastern San Juan Basin: a play limited by structure and associated thermal maturity

AAPG Rocky Mountain Section Meeting

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The Mancos Shale in the southeastern San Juan Basin: a play limited by structure and associated thermal maturity

Abstract

Oil has been produced from northwestern Sandoval County at the southeastern end of the San Juan Basin since the 1950’s. Cumulative production is 19.2 MMBO. The Mancos Shale (Upper Cretaceous) is the primary producing unit. Most Mancos oil has been produced from the Mancos C zone at the base of the Upper Mancos Shale. Horizontal drilling in the Mancos C caused a resurgence in production from 69.5 MBO in 2011 to 2.1 MMBO in 2015. The Mancos C is 400 to 600 ft thick. Productive reservoir intervals are laminations and thin beds of fine-grained marine shelf sandstones intercalated with kerogen-rich marine shales. TOC of the shales varies from 1 to 3% and generally increases in a northeast, offshore direction. Oil-prone kerogens dominate. In far northwestern Sandoval County where the Mancos C is 5400 ft deep, production has been established in150 wells. Ro varies from 0.85 to 1.04%. The Mancos C is at peak oil generation throughout most of this area. Depth to Mancos C is approximately 5400 ft in this area and the Mancos C is 400 to 600 ft thick. Southeastward as the Mancos C rises out of the basin, thermal maturity decreases. The Mancos C is immature near its outcrop in central Sandoval County. The southeast limit of Mancos C production coincides roughly with the transition from the lower the upper oil windows. Production is scattered in areas where peak oil generation has not been attained.