Abstract

The Lewis Shale Total Petroleum System (TPS) in the San Juan Basin contains a continuous gas accumulation in three distinct stratigraphic units: 1) the Lewis Shale, and 2) the La Ventana Tongue and 3) the Chacra Tongue of the Cliff House Sandstone. The Lewis was not a primary completion target in the San Juan Basin (SJB) in drilling from about the 1950s through 1990, when only 16 wells were completed from natural fracture systems encountered while drilling for deeper objectives. In 1991, existing wells that penetrated the Lewis were fracture stimulated to add Lewis production to pre-existing Mesaverde production. Based on recent gas production from the Lewis, and because new models indicate both source and reservoir rocks in the Lewis, the Lewis TPS was evaluated as part of the USGS oil and gas assessment of the San Juan Basin.

Lewis Shale TPS gas is produced from shoreface sandstones and siltstones in the La Ventana and Chacra Tongues and from distal facies of these same units that extend into marine rocks of the Lewis Shale in the central part of the San Juan Basin. The Lewis contains at least five high-order sequences that juxtapose organic-rich flooding surface marine shales with high-stand and transgressive systems tract reservoirs. The Lewis Continuous Gas Assessment Unit is self-sourced and self-sealing from marine shales and mudstones deposited along marine flooding surfaces that extend southwestward and overlie clastic rocks in the La Ventana and Chacra Tongues. The Lewis Continuous Gas Assessment Unit (AU 50220261) has an F95 of 8,315.22 BCFG (billion cubic feet of gas), and an F5 of 12,282.31 BCFG, with a mean value of 10,177.24 BCFG. There is an F95 of 18.08 MMBNGL (million barrels of natural gas liquids), and an F5 of 47.32 MMBNGL, with a mean of 30.53 MMBNGL.