A tale of twopoint bars: inclined heterolithic strata and reservoir compartmentalization, Piceance Basin, Colorado

Patrick Boulas, Kimberly Hlava, Matthew Pranter, and Rex Cole

Detailed analysis of inclined heterolithic strata (IHS) associated with fluvial-dominated point bars and a tidally influenced fluvial point bar was conducted to compare internal heterogeneity and potential compartmentalization. Well-exposed outcrops of the Mesaverde Group of this study are located in the northern Douglas Creek Arch, Colorado, and approximately 20 mi (32.3 km) west of productive reservoirs from approximately the same stratigraphic interval in the Piceance Basin. Compartmentalization of the equivalent fluvial reservoirs is commonly due to internal lithological and lithofacies variability of point bars which can potentially impact hydrocarbon sweep efficiency, productivity, and ultimate recovery. Sedimentological analysis and paleocurrent orientations of fluvial and tidal point bars aid in distinguishing depositional environments. The tidally influenced point bar contains abundant low-energy sedimentary structures (ripple cross stratification with subordinate planar cross stratification). Fluvial point bars contain abundant high-energy facies (trough and planar cross stratification with subordinate ripple cross stratification). Paleocurrent measurements in both point bars show internal cross stratification perpendicular to the lateral-accretionary sets, however, the tidally influenced point bar contains abundant bi-directional cross stratification whereas the fluvial point bar contains uni-directional cross stratification. The geometry and connectivity of IHS are influenced by depositional processes. Both fluvial and tidally influenced point bars are lenticular in two dimensions. The observed tidally influenced fluvial point bar exhibits continuous mudstone-dominated drapes between sandstone lateral-accretion deposits. The drapes create significant internal compartmentalization within the sandstone bodies (potential reservoir elements). For the majority of the observed fluvial-dominated point bars, preservation of mudstone drapes between lateral-accretion deposits is variable, and commonly the mudstone drapes are preserved only on the upper portions of accretionary units. As a result, internal connectivity of the fluvial-dominated point bars can be somewhat high relative to the tidally influenced fluvial point bars. Based on petrographic analysis and mini-permeametry measurements, permeability of the IHS for the tidally influenced deposits decreases down-dip within the units, thus adding to the internal heterogeneity.

AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012