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

Don Yurewicz
ExxonMobil Exploration Company

The Mesaverde Group in the Piceance Basin encompasses many of the attributes of a basin-center gas accumulation. A basin-center gas system is typically described as a regionally extensive gas accumulation situated in the central, deeper parts of a basin. Reservoir rocks are typically low porosity (< 13%), low permeability (<0.1 md) discontinuous sandstones (commonly fluvial). Gas distribution lacks any obvious relationship to structural closures or stratigraphic traps with downdip water contacts. According to the model, reservoir sands within the limits of the play are gas saturated with little or no producible water. The limits of a basin center gas accumulation may be indistinctly defined. Gas saturated reservoirs may grade vertically and laterally across stratigraphic boundaries into transitional water- and gas-bearing zones. Furthermore, laterally continuous blanket-like reservoirs with better connectivity and permeability than surrounding lenticular reservoirs may produce significant volumes of water. Observations from wells across the Piceance Basin confirm that the vertical extent the Mesaverde basin-center gas accumulation varies regionally and predictably across the basin, that there is a mapable transitional water- and gas-bearing zone, and that the Mesaverde includes reservoirs capable of producing large volumes of water. Obviously understanding the distribution of gas is critical to determining the resource potential of the Mesaverde Group, and in developing a successful exploration and production program. Understanding the distribution of water within the Mesaverde is equally important - disposing of water from gas zones with high water rates affects overall project economics and avoiding high water zones directly reduces ultimate gas recovery.

Several approaches have been taken to understand the distribution of fluids within the Mesaverde Group. They include modelling gas generation within the Piceance Basin, mapping the gross gas column based on mudlog shows, mapping the distribution of wells with high water production, and identifying reservoir zones with high water production. This work has been integrated with regional stratigraphic and fracture frameworks in order to understand the interplay of reservoir architecture (sand geometry and quality) and fractures on the movement of fluids within the basin. The result of these studies indicates that the distribution of gas within the Mesaverde Group reflects total gas yield and the ability of different sand bodies within the Mesaverde to trap and retain gas.

The reservoir and stratigraphic architecture of the Mesaverde Group and overlying Ohio Creek Conglomerate play an integral role in the distribution of fluids and are briefly described here. Four reservoir types are recognized in this section (Figure 1). They include marine shoreface sandstones, coastal-plain meandering-stream channel sandstones, distal braided-stream channel sandstones, and proximal braided-stream amalgamated channel sandstones. These sandstones are interbedded within marine to non-marine shales and minor coals and the ability to transmit fluids within the basin is strongly controlled by sand geometry and continuity. Marine shoreface sandstones are characterized by high lateral continuity and excellent connectivity. Individual sandstones are 10 to 100 feet thick and can be correlated on a basin scale. The coastal-plain deposits of the Corcoran-Cozzette and Cameo sections are characterized by meandering-stream channel sandstones. Channel sandstones are typically 10 to 20 feet thick and have a highly lenticular cross channel profile. Sand packages greater than 20 feet in thickness may well represent stacked, amalgamated channels or channel complexes, rather than single channels. The distal braided-stream channel sandstones of the Williams Fork Formation are lenticular and fairly discontinuous. In contrast to the meandering-channel sandstones, distal braided-stream sandstones do not show the internal, lateral accretion surfaces of typical meandering stream point bars that may further restrict lateral fluid flow. The uppermost Williams Fork Formation and Ohio Creek Conglomerate are highly amalgamated, stacked proximal braided-stream channel deposits. These multi-story channels are laterally very continuous and highly connected. These sandstones are coarser-grained and are characterized by higher porosities and permeabilities than other sandstones in the Mesaverde Group.

The very low permeability of Mesaverde reservoirs indicates that long-distance migration of fluids (gas and water) is largely controlled by fractures. Fractures are best developed in competent regionally continuous proximal braided-stream sandstones of the Ohio Creek Conglomerate and the uppermost Williams Fork Formation, and marine sandstones within the Rollins, Corcoran, Cozzette, Sego and Castlegate. Fractures in discontinuous fluvial channel sandstones within distal braided-stream and meandering-stream facies are characterized by lower spacing and lower vertical and lateral extent. This combined with the discontinuous lenticular geometry of the preserved channels and 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 discontinuous distal braided-stream and meandering-stream fluvial channels while the more continuous marine sands and amalgamated proximal braided stream sands are conduits for migration of gas out of the basin and recharge of water into the subsurface. Test data from across the basin confirm these relationships. High water rates are especially a risk for the amalgamated proximal sandstones of the upper Williams Fork Formation and Ohio Creek Conglomerate and the thick, widespread shoreface sandstones of the Rollin Member of the Iles Formation. While these reservoirs are locally good gas producers, they are most likely to produce large volumes of water with gas.

The top of continuous gas shows in wells across the basin is a fairly accurate representation of the top of productive gas with low water yields. Data from ExxonMobil and publicly available mudlogs were used to map the top of gas basin-wide. These data were then converted to an isopach of the column of gas saturated sandstones by mapping the interval between the top of gas and the top of the Rollins Sandstone within the Iles Formation (Figure 2). The Rollins was used because it is a consistent mapping surface throughout the basin. The resulting map (Figure 2) and cross-section (Figure 3) show that the Mesaverde gas interval is thickest along northern axis of the basin and decreases to the flanks. Continuous gas shows extend more than 3,000 feet above the top of the Rollins along the northern axis of the basin and are below the top of the Rollins on the western flank of the basin.

Gas is present in sandstones above this surface but gas saturations decrease and there is a high risk for high water production. These reservoirs constitute what has been referred to as the transition zone of the Mesaverde basin-center gas play. In nearly all areas, the top of gas occurs below the base of amalgamated proximal braided-stream sandstones of the uppermost Williams Fork and Ohio Creek formations suggesting that these sands are likely conduits for fluid migration out of the basin.

There is a fairly consistent relationship between the height of the "gas column" and the volume of gas generated from Mesaverde source rocks suggesting that the distribution of gas within the basin is, at least in part, controlled by gas charge (Yurewicz et al., 2003). A series of twenty-seven burial history models were generated for wells across the Piceance Basin and gas yields were determined from interbedded Mesaverde source beds and from source beds in the underlying Mancos Shale. There are three source facies within the Upper Cretaceous section in the Piceance Basin. These include (1) marine shales within the Mancos Shale and Iles Formation at the base of the section, (2) floodplain shales within the coastal-plain and braid-plain facies of the Iles and Williams Fork formations, and coals within the coastal plain to distal braid-plain facies of the Iles and lower Williams Fork formations. Yield modelling indicates that coals are the most significant source of gas within the Mesaverde section. Gas generation was greatest near the deep axis of the basin in the north and reflects greater burial depth and maturity of source beds. Although structural dip of Mesaverde beds towards the basin center and pervasive regional fractures suggest there are potential flow paths to charge all portions of the study area the high level of correlation between gas generation and height of the "gas column" suggest that low permeabilities and the highly discontinuous geometry of most Mesaverde reservoirs inhibit lateral migration. Lateral migration, instead, appears to be restricted to laterally continuous marine and proximal braided-stream reservoirs.

References

Yurewicz, D. A., K. M. Bohacs, J. D. Yeakel, and K. Kronmueller, 2003, Source rock analysis and hydrocarbon generation, Mesaverde Group and Mancos Shale, northern Piceance Basin, Colorado, in Peterson, K. M., Olson, T. M., and Anderson, D. S., eds., Piceance Basin 2003 Guidebook: Rocky Mountain Association of Geologists, p. 130-153.

Figure 1. Depositional and stratigraphic framework of the Piceance Basin.

Figure 2. Isopach of the "gas column" (based on continuous gas shows) above the Rollins Sandstone.

Figure 3. East-west cross-section through the northern Piceance Basin showing the six major reservoir intervals and mapped top of gas.