Luncheon – April 5, 2002

Title: Sequence Stratigraphy of the Dakota Sandstone, Eastern San Juan Basin, New Mexico, and Its Relationship to Reservoir Compartmentalization

Speaker: Peter J. Varney, PhD, the Metropolitan State College of Denver

Date: April 5, 2002

Publication: The Outcrop, April 2002, p. 4

Dakota outcrops in the eastern San Juan Basin, New Mexico, show that the interval contains three depositional sequences within an overall third-order transgressive systems tract. Sequence 1 lies on the regional K2 unconformity above the Early Cretaceous Burro Canyon Formation and extends upward to Dakota Surface 2 (S2), a combination lowstand surface of erosion/transgressive surface of erosion that was subaerially exposed. Sequence 2 extends from S2 upward to Surface 3 (S3), a regressive surface of erosion that extends throughout the study area and perhaps much further. The interval between S2 and S3 comprises a transgressive systems tract in which there are two or more shorefaces that prograded basinward during fourth-order relative sea level fluctuations. Sequence 3 extends from S3 upward to the top of the Dakota interval. It contains fourth-order lowstand, transgressive and possible highstand systems tracts (HST).

Dakota parasequence characteristics are continuous over great distances along paleostrike, allowing outcrop gamma ray profiles to tie to, and permit calibration of, well logs in the South Lindrith Field, 44 miles to the southwest. Subsurface well log cross sections shaw that the sequence framework established on outcrop applies to the subsurface.

Within the Sequence 3 HST, northwest to southeast trending shoreface deposits prograded northeastward. Syndepositional faults, detected through very close-spaced contour mapping, helped determine their orientation. One of these shoreface deposits produces gas from abundant porosity in a fault bounded, stratigraphic reservoir compartment. It helps confirm the importance of structurally enhanced reservoir compartmentalization in the Rocky Mountain area.