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Developing a Compound Morrow Valley Fill: Reservoir Heterogeneity and Implications for Reservoir Management and Field Expansion, State Line Trend, Colorado/Kansas
Integrated analysis of over 2,000 well logs calibrated with 120 cores in the Lower Pennsylvanian Morrow Formation has yielded a detailed subsurface example of the complex sedimentology and sequence stratigraphic relationships of incised valley fills and their associated sequence boundaries. Throughout Morrowan time the study area in eastern Colorado and western Kansas was a broad, flat (0.005° gradient), muddy shelf, fully flooded during highstands and dissected by numerous fluvial valleys during lowstands. The Transcontinental Arch to the northwest and the Central Kansas Uplift to the east provided coarse, quartzose sediment to streams which flowed south to shelf-edge deltas in the Anadarko Basin of Oklahoma.
The Morrow Formation contains at least 9 distinct, regionally-extensive Type I sequence boundaries inferred to record glacio-eustatic cyclicity with a frequency on the order of 100 to 200 Ka. These sequence boundaries are marked by narrow (0.5–2 mi., 0.8–3.2 km) sinuous fluvial incisions which occur along low-relief exposure surfaces on marine shale and limestone. Most of the production from Morrow sandstone reservoirs is from lowstand and transgressive deposits within these 50–80′ (15–24.5 m) deep valleys. Highstand systems tracts are rare to absent in this area because of ravinement or non-deposition.
Sedimentologic and stratigraphic complexity are caused by the close temporal and stratigraphic spacing of the sequence boundaries and flooding surfaces. Along the State Line Trend a stacked set of 4 superimposed valleys has been documented. Each of these valleys is distinctive in its grainsize, sedimentology and well-log characteristics. Although the successively younger valleys can incise into and amalgamate with older valley deposits, integrated core and log data allowed differentiation and mapping of each successive valley fill. This mapping proved critical to reservoir management, documented new development locations and indicated new directions for exploratory drilling.
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