Print this pageComparison of Basin-Scale Clinoform Packages of the Lewis Shale, Wyoming, Usa, to Shelf-Scaled Clinoforms of the Lower Nutovo Formation, Chayvo Field, Sakhalin I Area, Russia
The Lewis Formation of Wyoming includes a clinoforming succession prograding into a small deepwater basin in the Cretaceous interior seaway. Comparison to Miocene-age reservoirs of the Chayvo Field, Sakhalin area, Russia reveals similarities in clinoform-associated depositional processes but differences in scale and duration that relate to shelfal versus basin-margin accommodation.
Boundary conditions at the time of deposition of both the Chayvo and Lewis clinoforms were similar with subsidence rates, sediment accumulation rates, and progradation rates being nearly equal. One key difference is that the Lewis depositional sequences prograded to and constructed the shelf margin. Chayvo Field clinoforms formed on a broad depositional shelf with the Miocene continental margin scale shelf break was located to the east.
The main reservoirs at Chayvo Field are found within a highstand sequence set of 6.6 my duration, encompassing 11 depositional sequences. One well-documented and seismically-defined sequence is called Zone XVII, age-dated at 11.7 to 12.7 ma (Upper Miocene). Most of the seismically defined reservoir-bearing clinoforms are found within the highstand systems tract of 600ky duration. Nine individually mappable clinoforms are present within this parasequences set. Stacking patterns are characteristic of highstand systems tracts: early clinoforms are aggradational, later clinoforms are progradational. Clinoforms are only 30m thick early in the early highstand but reach >100m compacted thickness late in the highstand. EOD’s and lithologies follow timelines and clinoforms in general, with the east side of the field containing most of the reservoir volume. Lack of slump structures in cored wells suggests that slopes were not as great as observed in the Lewis Formation (<3 degrees. Cores do indicate significant wave influence in Chayvo, a likely function of significant fetch in the paleo-Pacific ocean.
Another similarity between Chayvo Field reservoirs and the Lewis Formation lies in the presence of small confined, slightly sinuous channels extending down clinoforms and filled with relatively thin sandstones. Outcrops of the Dad Member of the Lewis at Spine 1, previously reported by other workers, contain Ta/Tb beds indicative of suspension deposition from high concentration turbidity flows. Channels at Chayvo are usually less than 100 m in width and 10 m in thickness, comparable to the Lewis at Spine 1.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009