Unconformities and Valley-Fill Sequences--Key to Understanding Reservoirs at Lonetree and Poncho Fields, Denver Basin, Colorado

Frank Ethridge, J. C. Dolson

Previous interpretations suggested that the Lower Cretaceous "J" sandstone in the Denver basin is divisible into three genetic units: a lower delta front (J-3), a middle delta plain (J-2), and an upper destructional marine bar (J-1). The presence of root casts and siderite cement along the upper contact of the J-3 sandstone, however, suggests that J-2 deposits are separated from the J-3 sandstone by a regional unconformity. This contact may mark the same unconformity that separates the Horsetooth and Fort Collins Members in outcrop along the western margin of the basin. J-2 deposits comprise point bar, crevasse splay, and floodplain sequences of a broad alluvial plain. Fluvial deposits at Lonetree and Poncho probably formed in a tributary to a larger order tributary that lowed southwest through the Peoria field area. These tributaries eventually drained into a major trunk stream represented by valley-fill deposits of the Kassler sandstone.

Delta-front deposits (J-3) that underlie the unconformity are tightly cemented by siderite and do not produce in either field. J-2 sandstones are productive from north- and west-trending point-bar deposits in both fields and from laterally equivalent crevasse-splay deposits at Lonetree field. Secondary porosity has created a common reservoir in point-bar and crevasse-splay deposits at Lonetree. Laterally equivalent deposits of major trunk streams, such as the Kassler sandstone, generally produce only over structural noses in the southern D-J basin. Overlying J-1 sandstones form northeast-trending marine bars in both field areas. Two echelon bars are present at Poncho but only the western one is productive. The eastern bar at Poncho and the single marine bar at Lonetree are wet.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.