Controls on Gas and Water Distribution, Mesaverde Basin-centered Gas Play, Piceance Basin, Colorado

Abstract

The objectives of this study are to characterize the distribution and production of gas and water within the Mesaverde basin-centered gas play in the Piceance Basin and to determine the extent by which they are affected by variations in hydrocarbon charge and basin hydrodynamics. Several approaches have been taken in this study to understand the distribution of fluids within the Mesaverde Group. They include (1) modeling gas generation within the Piceance Basin, (2) mapping the gross gas column based on mud-log shows, (3) mapping the distribution of wells with high water production, and (4) identifying reservoir zones with high water production. This work has been integrated with the regional stratigraphic framework and fracture distribution to understand their influence on the movement of fluids within the basin. The results of these studies indicate that the distribution of gas within the Mesaverde Group reflects both total gas yield and the ability of different sandstone bodies within the Mesaverde to transmit and/or trap and retain gas. Basin modeling indicates that coals within the Iles Formation and the lower part of the Williams Fork Formation have generated the largest volume of gas. Organic-rich continental shales, although thick and present throughout the Mesaverde Group, generally have low hydrogen indices and have generated comparatively small volumes of gas. Marine shales at the base of the section have slightly higher hydrogen indices than continental shales within the Mesaverde Group, but have moderately low total organic carbon and have also generated smaller volumes of gas than the coals. Basin modeling indicates that gas generation was greatest in or near the deep axis of the basin in the north and reflects greater thermal maturity of source beds. A fairly consistent relation exists between the volume of gas generated from Mesaverde source rocks and the height of the gas column within the Mesaverde, suggesting that the distribution of gas within the basin is, at least in part, controlled by gas charge. The Mesaverde gas column is thickest along the northern half of the basin axis and decreases to the flanks, consistent with variations in total gas generation. Produced water also varies relative to the top of the mapped gas. The very low permeability of Mesaverde reservoirs indicates that long-distance migration of fluids (gas and water) is largely controlled by fractures. Natural fractures in the Piceance Basin are strongly parallel; hence, connectivity depends mainly on fracture density, length, and height. Most fractures terminate at sandstone body margins; hence, discontinuous fluvial-channel sandstones within distal braided-stream and meandering-stream facies are characterized by lower fracture connectivity than more continuous marine sandstones and amalgamated proximal braided-stream sandstones. This, combined with their very low permeability, results in low fluid mobility. These sandstones are less likely to leak gas to the surface or to act as recharge conduits for surface water. Hence, gas is preferentially trapped in poorly amalgamated distal braided-stream and meandering-stream fluvial channels, whereas the more continuous marine sands and amalgamated proximal braided-stream sandstones are conduits for migration of gas out of the basin and recharge of water into the subsurface.