John W. Robinson

Factors controlling prolific gas production from low-permeability sandstone reservoirs: Implications for resource assessment, prospect development, and risk analysis

Low-permeability reservoirs from the Greater Green River basin of southwest Wyoming are not part of a continuous-type gas accumulation or a basin-center gas system in which productivity is dependent on the development of enigmatic sweet spots. Instead, gas fields in this basin occur in low-permeability, poor-quality reservoir rocks in conventional traps. We examined all significant gas fields in the Greater Green River basin and conclude that they all occur in conventional structural, stratigraphic, or combination traps. We illustrate this by examining several large gas fields in the Greater Green River basin and suggest that observations derived from the Greater Green River basin provide insight to low-permeability, gas-charged sandstones in other basins. We present evidence that the basin is neither regionally gas saturated, nor is it near irreducible water saturation; water production is both common and widespread. Low-permeability reservoirs have unique petrophysical properties, and failure to fully understand these attributes has led to a misunderstanding of fluid distributions in the subsurface. An understanding of multiphase, effective permeability to gas as a function of both varying water saturation and overburden stress is required to fully appreciate the controls on gas-field distribution as well as the controls on individual well and reservoir performance. Low-permeability gas systems such as those found in the Greater Green River basin do not require a paradigm shift in terms of hydrocarbon systems as some have advocated. We conclude that low-permeability gas systems similar to those found in the Greater Green River basin should be evaluated in a manner similar to and consistent with conventional hydrocarbon systems.To date, resource assessments in the Greater Green River basin have assumed a widespread, continuous-type resource distribution. Failure to recognize some of the fundamental elements of low-permeability reservoirs has led to an underappreciation of the risks associated with exploration and development investment decisions in these settings and likely a significant overestimation of available resource levels.

Sandstone-Body and Shale-Body Dimensions in a Braided Fluvial System: Salt Wash Sandstone Member (Morrison Formation), Garfield County, Utah

Excellent three-dimensional exposures of the Upper Jurassic Salt Wash Sandstone Member of the Morrison Formation in the Henry Mountains area of southern Utah allow measurement of the thickness and width of fluvial sandstone and shale bodies from extensive photomosaics. The Salt Wash Sandstone Member is composed of fluvial channel fill, abandoned channel fill, and overbank/flood-plain strata that were deposited on a broad alluvial plain of low-sinuosity, sandy, braided streams flowing northeast. A hierarchy of sandstone and shale bodies in the Salt Wash Sandstone Member includes, in ascending order, trough cross-bedding, fining-upward units/mudstone intraclast conglomerates, single-story sandstone bodies/basal conglomerate, abandoned channel fill, multistory sandstone bodies, and overbank/flood-plain heterolithic strata. Trough cross-beds have an average width:thickness ratio (W:T) of 8.5:1 in the lower interval of the Salt Wash Sandstone Member and 10.4:1 in the upper interval. Fining-upward units are 0.5-3.0 m thick and 3-11 m wide. Single-story sandstone bodies in the upper interval are wider and thicker than their counterparts in the lower interval, based on average W:T, linear regression analysis, and cumulative relative frequency graphs. Multistory sandstone bodies are composed of two to eight stories, range up to 30 m thick and over 1500 m wide (W:T > 50:1), and are also larger in the upper interval. Heterolithic units between sandstone bodies include abandoned channel fill (W:T = 33:1) and overbank/flood-plain deposits (W:T = 70:1). Understanding W:T ratios from the component parts of an ancient, sandy, braided stream deposit can be applied in several ways to similar strata in other basins; for example, to (1) determine the width of a unit when only the thickness is known, (2) create correlation guidelines and maximum correlation lengths, (3) aid in interpreting the controls on fluvial architecture, and (4) place additional constraints on input variables to stratigraphic and fluid-flow modeling. The usefulness of these types of data demonstrates the need to develop more data sets from other depositional environments.