New Mexico Geological Society Annual Spring Meeting — Abstracts

The Menefee Formation is generally regarded to be nonmarine in origin, underlain by the regressive marine Point Lookout Sandstone and overlain by the transgressive marine Cliff House Sandstone. The three formations comprise the Mesaverde Group. Most previous research of the Menefee Formation has focused on the coal and humate deposits mined today in the southern part of the San Juan Basin. In the northern part of the basin, sandstones in the Menefee are potentially important underdeveloped natural gas reservoirs. Their complex depositional facies, stratigraphy and reservoir characteristics have been poorly understood. A 147 m stratigraphic section was measured on the eastern hogback of the basin north of Regina, New Mexico, 1.2 km north of highway 96. A detailed description of the strata, focusing on the sandstone beds, was compiled.

Four major lithofacies represent distal floodplain (coastal swamp to marsh) to deltaic deposition. Lithofacies "Jm-1" (21 % of total) consists of laminated gray mudstone, interpreted to represent distal deltaic (subaqueous) inter-distributary channel deposition. Lithofacies "Jm-2" (25% of total) is dominated by brown mudstone and coally mudstone to humate, interpreted to represent overbank swamp to coastal marsh deposition. Lithofacies "Jm-3" (12% of total) comprises planar bedded sandstone, interpreted to be crevasse splay deposits in part; and 4) Lithofacies "Jm-4" (42% of total) is trough cross-bedded, channelized sandstone, interpreted to have been deposited in deep and narrow deltaic distributary channels. Some channel sands are Thalassinoides-burrowed, interpreted to indicate distributary channel backflooding (estuarine environment).

Petrography of six thin sections from sandstone beds yields point-counted framework grain composition of quartz (46%), chert (30%), feldspar (4%), and other rock fragments (3%). Chert is anomalously high compared to published data. Clay matrix averages 14% and cement 3%. Two major cement associations are observed, including coarse crystalline (poikilotopic) calcite-glauconite-dolomite-siderite-hematite (replacing dolomite) in lithofacies Jm-4; and fine-crystalline calcite-glauconite-hematite in lithofacies Jm-3. Gamma ray measurements reveal that the Menefee sandstone beds have higher counts than bounding formations, particularly high in the sideritic sandstone. Glauconite, and authigenic and detrital dolomite, both potentially of marine origin, are recognized in interpreted estuarine sandstone beds. A bentonite bed in coal at the top of the formation yielded a sanidine ⁴⁰Ar/³⁹Ar age of 78 ±0.26 Ma (2σ error).

Although further work is required, it is possible that, in contrast to previous published research, more distal depositional models should be applied to the Menefee Formation in the northern, gas-productive, part of the San Juan Basin than have been described in the southern part of the basin. This could have important implications for exploration and development of this resource as increased-density drilling proceeds.