Sedimentology and sequence stratigraphy of the Muddy (J) Sandstone in the Golden/Morrison area, Jefferson County and Peoria field, Arapahoe County, Colorado

The Muddy (J) Sandstone of the Lower Cretaceous Dakota Group is a prolific producer of hydrocarbons in the Denver basin. There is a large body of work on the Muddy (J) Sandstone (Weimer 1970, 1976, 1978, 1980, 1983, 1984, 1992, 1996; MacMillan, 1974; Clark, 1978; Poleschook, 1978; Suryanto, 1979; Altschuld, 1980; Sonnenberg, 1981; Grube, 1984; Young, 1987),) but the facies models have not been updated since the 1980's (Weimer, 1983). The focus of this thesis is (1) detailed outcrop documentation of sedimentary facies for a new depositional model, (2) comparison to near-outcrop core and well logs, and (3) comparison to core and well logs in Peoria field. Eight outcrop sections and six cores in their vicinity were described in detail. Nineteen cores and eighty three well-logs from the Peoria oil field were analyzed. This study resulted in new depositional models for the Muddy (J) Sandstone in the outcrop belt as well as the Peoria field. The Muddy (J) Sandstone in the outcrop belt is interpreted as an incised valley-fill system due to basinward shift of facies across the basal sequence boundary. The valley fill is composed almost entirely of tidal or tide-influenced deposits. This study identifies the Muddy (J) Sandstone as a tide-dominated estuarine system, based on the presence of the following facies associations: tidally-influenced fluvial deposits (Facies Association 1), fluvial-tidal channel deposits (Facies Association 2), outer estuarine axial tidal flats and channel deposits (Facies Association 3) which include: outer estuarine tidal channel deposits (Facies Association 3A) and upper flow regime tidal flats (Facies Association 3B), tidal sand bar deposits (Facies Association 4), sandy to mixed tidal flats (Facies Association 5), and mixed to muddy tidal flats and marshes (Facies Association 6). Fluvial facies associations occur in the northern part of the outcrop belt. In cores, these tidally-influenced fluvial deposits occur in the middle part of the Muddy (J) Sandstone. This is in contrast to the previous facies models, where the whole Peoria field succession was assigned to fluvial deposition (Ecker, 1971; Land & Weimer, 1978; Chapin, 1989). In addition to the fluvial deposits, this study identifies tidal deposits, and interprets the Peoria Field as a tide-dominated estuary, or tide-dominated delta.;Outcrop deposits of the Muddy (J) Sandstone show a landward stepping to aggradational stacking pattern. In Peoria field, a progradational to retrogradational stacking pattern is documented. Important differences between outcrop and subsurface deposits are the absence of tidal sand bars in cores of the Peoria field and the presence of more marginal muddy facies towards the top of the Muddy (J) Sandstone associated with the drowning of the estuary. Based on this study, it is recommended to explore for tidal sand bars which have better reservoir quality further seaward. Relative sea level changes as well as forebulge migration are inferred to be the main controls on the deposition of the Muddy (J) Sandstone.