Sedimentology and Stratigraphy of the Muddy (J) Sandstone, Wattenberg Field, Denver

Basin, Colorado

By

Edmund R. "Gus" Gustason, Stephen A. Sonnenberg, and Terry Patterson

EnCana Energy Resources Inc, Denver, CO

The Lower Cretaceous Muddy (J) Sandstone has produced nearly 1.0 TCF of gas from the basin-center Wattenberg Gas Field. Maps of the estimated ultimate recovery (EUR) from the Muddy reveal broad, regional trends punctuated by an irregular distribution of “sweet spots” and “dead spots”. To better understand the distribution of EUR “sweet spots” and better predict well performance, EnCana Energy Resources Inc is conducting a core-based, sequence stratigraphic and structural study of the Muddy Sandstone. Preliminary results are consistent with existing interpretations, but reveal a more complex interplay among depositional processes, syndepositional faulting and pedogenesis, and post-depositional faulting and diagenesis that helps explain mapping and reservoir performance anomalies.

The Muddy is divided into two members, the Fort Collins Member and Horsetooth Member. The older Fort Collins Member has a gradational contact with the underlying Skull Creek Shale and is unconformably overlain by the Horsetooth Member and/or Mowry Shale. The Fort Collins Member consists of several, eastward off lapping, upward coarsening marine shoreface deposits comprising a progradational parasequence set. The Horsetooth Member rests unconformably on the Fort Collins Member, separated everywhere by a lowstand surface of erosion (LSE). Several NW- and NE-trending valley tributaries were eroded into the composite Fort Collins Member strandplain. Across areas of thin or no valley incision, the upper ten feet of the Fort Collins Member is typically lighter colored (bleached), has an irregular distribution of silica, kaolinite, and siderite cement, and root traces, interpreted to represent soil development. The Horsetooth Member consists of fluvial-estuarine valley-fill deposits, bay-fill deposits, landward stepping, progradational (retrogradational) barrier island deposits, and transgressive marine deposits. Gross interval isopach maps indicate northeast trending wrench faults and northwest trending antithetic faults were active during the development of the intraformational unconformity, controlling Horsetooth paleovalley trends and the “preserved” thickness of the Fort Collins strandplain.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming