Tectonic and Sedimentation Model for D Sandstone Deposition, Zenith Field Area, Denver Basin, Colorado: ABSTRACT

Abstract

Cretaceous sandstones are oil and gas productive throughout a large area in the Denver basin. The Zenith field is a recently developed area that contains significant reserves in the D sandstone. Minor production also comes from the J sandstone. Detailed mapping of the D sandstone suggests that productive sandstones are of channel origin within a valley-fill complex. Trapping of petroleum appears to be mainly stratigraphic with structure playing a minor role. The stratigraphy of seven Cretaceous stratigraphic intervals was analyzed to determine if paleostructure may have influenced D depositional patterns. Thickness variations within stratigraphic intervals are caused by unconformities, convergence, and normal faulting. Thickness variations caused by unconformities and convergence may be related to paleostructure; variations caused by normal faulting are postdepositional and related to Laramide structure. Analyses of seven stratigraphic intervals clearly show that paleostructure influenced D sandstone depositional patterns. A new model proposed for D sandstone deposition incorporates paleotopography and sea level changes. During or immediately after the deposition of the Huntsman Shale and a thin regressive D sandstone deposit, a structura low area formed. The low area was probably created by basement fault-block movement. Concurrent with the tectonics, a drop in sea level occurs which drains a portion of the D depositional basin. A drainage system develops and follows the low area and incises through the regressive D deposit and into the Huntsman Shale. A sea level rise occurs and thick D channel sandstones are then deposited within the End_Page 949------------------------------ eroded valley. The present structure at the top of the J sandstone (stratigraphically older than the D sandstone) is a structural low in the area where D valley-fill sandstones occur. The trend and location of the low at J level are identical to the trend and location of the D valley-fill deposits. Thus, the present low at the J level confirms the paleostructure interpretation. This new model for D sandstone deposition, incorporating paleotopography and sea level changes, provides a new idea for petroleum exploration in the Denver basin. End_of_Article - Last_Page 950------------