Integrated Outcrop and Subsurface Evaluation of Permian Reservoir Trends and Paleostructural Controls on Facies and Reservoir Rocks, Eastern Big Horn Basin, Wyoming, U.S.A.

Abstract

Regional stratigraphic cross-sections and mapping of Phosphoria-age formations link outcrop sections (including twelve new measured sections at Sheep Mountain) with almost 100 cored wells. These correlations reveal complex large- and small-scale Permian stratigraphic changes caused by both paleotectonics and four major (and many minor) sea-level cycles.

A northwest-oriented Phosphoria strandline is marked by peritidal barrier islands and tidal flats that a.) initiated during sea-level high-stands, b.) accumulated by progradation and filling of fault-related accumulation space by algal boundstones and pisolitic grainstones-packstones; and c.) developed teepee structures up to thirty feet high during low-stand exposures. Eastward, strandline facies grade into cyclic salina deposits, i.e., high-stand red and green shales and microbial carbonates and low-stand anhydrites and solution collapse breccias capped by thin purple phosphatic cherts that may represent maximum sea-level highs. Westward, strandline deposits grade into restricted-marine pellet packstones and wackestones that themselves grade both westward and vertically into mollusk-rich lime packstones and open-marine bryozoan-crinoid-brachiopod limestones and cherts.

Fault trends interpreted from high resolution aeromagnetic data and confirmed by seismic and subsurface data coincide with Phosphoria stratigraphic changes. Both the east-trending Tensleep fault zone and the northeast-trending Tongue River fault zone display repeated Permian movements that localized two major low-stand paleo-valleys and created east-northeast embayments in the Permian shoreline.

At Sheep Mountain outcrops less than 300 feet apart evidence a low-stand valley that truncated thirty feet of peritidal deposits and subsequently filled with restricted marine facies. At Cottonwood Creek Field repeated flood-then-exposure events are revealed by multiple subtidal-peritidal couplets.

Phosphoria strandline rocks have yielded hundreds of millions of barrels of oil production. Depositional facies relate closely to reservoir quality and to flow units. Highly porous and permeable fenestral fabrics can be found in peritidal rocks. Restricted marine reservoirs typically have fine-crystalline, moldic/vuggy porosity with low permeability. Peritidal rocks formed in lower intertidal settings are superior reservoirs where laminoid fenestrae developed and porosity was not lost to early cementation of originally aragonitic (now dolomitic) sediments.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018