--> Controls on Regional Distribution Patterns in Prolific Western Interior Shelf Sand Reservoirs: Tocito, El Vado and Gallup Sands of the San Juan Basin

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Controls on Regional Distribution Patterns in Prolific Western Interior Shelf Sand Reservoirs: Tocito, El Vado and Gallup Sands of the San Juan Basin

Abstract

The Tocito Sandstone (TS) has been long proven to be a highly prolific reservoir system in the largest domestic onshore conventional gas basin in the U.S., the San Juan Basin (SJB). Similar shelf sand types associated with the pre-Tocito Gallup Sands and the more transgressive post-Tocito El Vado Sandstones have proven equally productive in recent wells resulting in a mini-boom of sorts in the SJB. Studies of the nature of all these shelf sand systems from outcrop, core, logs and seismic reveal thick (1-2 m) cycles of heterolithic wave-rippled, moderately to intensely bioturbated marine sands stacked in 8-12 m thick shelf sequences that are spatially extensive throughout the eastern as well as western SJB. Tocito sands in the western SJB outcrop are much more proximal in nature with tidal channel and bar facies associations predominate. Tocito intervals in the southeastern SJB outcrop show at least six sanding and thickening upward cycles composed of thinly-laminated, wave-rippled sands (Facies 3) interbedded with marine shales (Facies 1) progressing upward to moderately bioturbated, sand-rich parasequences (Facies 4). Shelf parasequences compensationally stack in near paleo-shoreline regions around Cabazon Peak northward to subsurface localities at least 140 km north of the paleoshoreline. Near-shore cycles near Cabazon Peak transition just 8 kilometers northward to contain extensive mega-hummocks (Facies 2). Analysis of hummocks in the Tocito sands suggest mega-swell waves up to 9 m high may have impacted the paleo-shelf distributing sands widely across the region, and possibly contributed to the submarine erosion of up to 60 meters of material from the paleo-shelf. This regional study of SJB shelf reservoir sands is the first to quantify the regional nature of sand distribution, link super-greenhouse processes to potential shelfal submarine erosion and redistribution of sediments, and to examine prograding versus transgressive shelf reservoir systems.