Low net:gross reservoir trends and exploration potential of the El Vado Sandstone Member of the Mancos Shale, northwestern New Mexico

Lesli Wood and Tiffany Hedayati

The El Vado Sandstone Member (EV) of the Mancos Shale, considered by many to be an unconventional hydrocarbon resource target, was studied to define it's age, sequence architecture, depositional environment, extent, sedimentology and origin with hopes of improving exploration and exploitation opportunities in this productive interval of the San Juan Basin(SJB). The El Vado is reflected in logs as a thick (~120') interval of low gamma ray and high resistivity. The gamma ray log character reflects highly laminated sand-to-silts interbedded with shales. Outcrop work in the NE SJB show the EV to be composed of five cycles, each ~4 m thick, separated by marine shales with the overall succession sanding up. Siltstones and very fine-grained, calcareous, and locally rippled or cross-bedded, sands are interlaminated with shale units. In southern outcrops, EV cycles are shelfal deposits capped by lower shoreface. Sandstone beds are 10-50 cm thick and contain shell fragments, burrows and some thin organic layers. The uppermost beds are very fine- to fine-grained sands showing large burrows, ripples and swaley and hummocky cross-stratification. Over 400 subsurface logs enabled correlation of cycles and generation of isopachs for the entire interval and individual EV cycles. Regional net sand and net-to-gross maps, determined from 104 digitized well logs, highlight the regional distribution of sand content. Regional work suggests the Lower EV cycles 1, 2 and 3 to be associated with the more westward Tocito 1 and 2 regressive sands and the Upper EV cycles 4 and 5 to be associated with the Tocito 2 and 1 transgressive sands. Evidence suggests the southern EV was deposited in a storm-influenced, marine shelf shallowing up to a lower shoreface. To the north, the EV appears to have been deposited in a more distal-shoreface to shelfal setting. Improved sand content to the south, as well as regional hydrocarbon migration pathways suggest enhanced exploration success might be had by expanding drilling to the south and west. Compensated deposition patterns in the EV provide the opportunity for stacked pays and show that depositional topography on the Mancos shelf had a strong influence on the occurrence of 'EV-like' sands. Such topography should be taken into account when exploring for sweet spots within the Mancos shelf succession. Finally, the El Vado core show enhanced natural fracturing in the shalier intervals, a point which requires more investigation.

AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012