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Significant accumulations of oil and gas are stratigraphically trapped in the Lower Cretaceous "J" sandstone in the southern Denver-Julesburg basin of Colorado. Lonetree Field (T3S, R59W) provides a typical example of trapping mechanisms and reservoir properties. Examination of 5 cores and 75 wire-line logs within the field resulted in several conclusions.
The deltaic J sandstone at Lonetree can be divided into three genetic units; a lower delta front (J-3), a middle delta plain sequence (J-2), and overlying destructional marine (J-1). Production at Lonetree is primarily from channel and crevasse splay sandstones of the J-2 interval. Traps are formed by the updip pinch-out of quality sandstone.
Channel sandstones are characterized by a coarse to very fine-grained upward-fining sequence. Trough cross-beds predominate at the base, and ripples and rootlets at the top. Carbonized wood is ubiquitous throughout. These northeast-southwest-trending channels attain thicknesses of 15 to 50 ft (4.5 to 15 m) and widths up to 2,500 ft (760 m).
Crevasse splay deposits show extreme lateral and vertical variation. Both coarsening and fining upward deposits are possible. Maximum thicknesses of 15 ft (4.5 m) are developed only in close proximity to channel sandstones. Rooted zones and ripple cross-stratification are common.
Porosity in both channels and splays is secondary in nature, resulting from the dissolution of feldspars and calcite cement. This overprinting of secondary porosity on a complex depositional system has created numerous separate reservoirs within the field. Porosity in producing zones is commonly 13 to 20%, with permeabilities in excess of 75 md. Kaolinite is abundant in pore throats, and may present completion problems associated with brushpiling of fines during treatment.
Little petrographic or petrophysical differences appear to exist between productive splay and channel sandstones. Typical cumulatives to date are 100,000 bbl of oil per well from splay sandstones, while channels contribute 125,000 bbl of oil per well. The 17 producing wells are expected to yield 2.4 million bbl of oil and 4 bcf of gas.
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