Porosity, Permeability, and Pore Structure of the Tight Mesaverde Sandstone, Piceance Basin, Colorado

Abstract

Summary Special core analyses on 44 tight Mesaverde sandstone samples from the U.S. DOE Multiwell Experiment (MWX) were combined with petrographic investigations to relate the porosity and permeability petrographic investigations to relate the porosity and permeability of the cores to the pore structure of the rocks. Core analysis was performed on 1-in. [2.54-cm] -diameter horizontal plug samples with a performed on 1-in. [2.54-cm] -diameter horizontal plug samples with a computerized steady-state-flow measuring device that routinely measures gas flow rates with a resolution of better than 10–6 std cm3/s. All samples were selected from intervals expected to be gas-productive on the basis of wireline well logs and were taken from the portion of the interval that showed the lowest gamma ray log response. The portion of the interval that showed the lowest gamma ray log response. The core plugs were measured for dry permeability to gas, relative permeability at various water saturations, porosity to gas, and PV compressibility. Petrographic samples were taken directly off the plug ends and were Petrographic samples were taken directly off the plug ends and were analyzed with both an optical microscope and a scanning electron microscope (SEM). The petrographic study was explicitly directed toward observing the flow paths and pore structure deduced from the core analysis data. Petrographic observations revealed that the pore geometry of tight sandstone falls into three general categories: grain-supported primary pores, secondary solution pores connected by narrow intergranular slots pores, secondary solution pores connected by narrow intergranular slots (the most common pore structure in the Mesaverde), and matrix-supported grains. Reservoir properties measured from core analysis correlated fairly well with the observed pore geometry and showed trends associated with various depositional environments in the formation. Introduction Over the past several years, the U.S. DOE has been conducting a field experiment designed to test the viability of lenticular and blanket tight sands as a gas resource. The MWX consists of three closely spaced wells on a site in the Piceance basin of western Colorado near Rifle. The stratigraphic unit targeted for research in the MWX is the Cretaceous Mesaverde group. The Mesaverde consists of nearly 5,000 ft [1525 m] of clastic sedimentary rocks at the MWX site and is divided into three formations. The Iles formation comprises the basal portion of the Mesaverde group. It is about 850 ft [260 m] thick at the wellsite and consists of three major blanket sandstone members (Corcoran, Cozzette, and Rollins) intertongued with and overlying the Mancos shale. The Williams Fork formation occurs above the Iles formation and is composed of about 3,280 ft [1000 m] of lenticular sandstones interbedded with siltstones, shales, and coal seams. The Fort Union formation is at the top of the Mesaverde group and consists of another 615 ft [185 m] of pebble conglomerates and sandstones. The Mesaverde group is, overall, a regressive sequence of sediments. As such, depositional environments change from offshore marine to terrestrial river floodplains as one progresses from the base to the top of the stratigraphic column. The following designations for depositional environments in the Mesaverde were generated by the MWX field geologists and will be used in this paper for consistency. Marine. Applied to the entire Iles formation. This zone consists of about 850 ft [260 m] of shoreface and near-shore shallow marine blanket sands intertongued with deepwater marine shales. Paludal. Applied to the coal-bearing lower 830 ft [255 m] of the Williams Fork formation in the MWX wells from the basal formation contact to the top of the upper-most major coal seam. This zone consists of lower-delta- plain lenticular sandstones interbedded with swamp coals plain lenticular sandstones interbedded with swamp coals and shales. Coastal. Applied to the noncoaly lower-middle portion of the Williams Fork formation. This zone is about 570 ft [175 m] thick in the MWX wells and consists of lenticular channel sands and shales deposited in a low-flow- regime meander belt along an upper-delta plain. Fluvial. The highest stratigraphic interval tested in the MWX comprises about 1,530 ft [465 m] of the upper half of the Williams Fork formation. Sediments in this zone consist of high-flow-regime lenticular sandstones (point bars, levees, and channel fill) interbedded with flood-plain siltstones and shales. Fig. 1 shows the stratigraphy of the Mesaverde group at the MWX site.