ABSTRACT: Nontectonic Structures Cause by Differential Compaction Over Buried Sand Bodies, Southern Sacramento Basin
FISCHER, PETE, California State University at Northridge and Mesa Verde E & P Services, Whittier, CA, and VIC CHERVEN,* Consultant, Shingle Springs, CA
Many folds and faults in the Sacramento basin die out with depth and exhibit little or no movement on the crystalline basement. Some of these features are due to differential compaction of sand and shale in underlying strata. Three examples of these structures are a closed anticline over a linear fluvial channel in the Mokelumne River Formation, a plunging anticline over a fan-shaped submarine-fan lobe in the Winters Formation, and a listric normal fault bordering a linear fan channel in the Winters Formation.
In the Mokelumne River example, a distributary-channel sand body trends southwest across two northwest-trending down-to-the-west normal faults. The sand body is overlain by a more widespread flood-basin lignite bed, and structure contours on this bed reveal a closed anticline centered on the axis of the sand body. The lignite bed thins toward this axis, and is absent in two wells located on the axis. Both wells were productive, with as much as 30 ft of closure over adjacent off-axis dry holes. Postdepositional compaction of overbank deposits adjacent to the distributary channel apparently allowed deposition of flood-basin peat, and that subsequent compaction and lignitization of the peat created a compactional anticline that trapped gas during later migration.
Winters fan lobes are thick, mounded sand bodies that broaden down depositional slope. In one example, the lobe is several miles across and over 500 ft thick along its axis. A low-resistivity shale marker bed overlies the lobe, and structure contours on this bed
reveal that a southwest-plunging anticline with as much as 150 ft of relief is centered on the axis of the fan lobe. The marker bed was deposited by hemipelagic fall-out over the mounded lobe surface, and underwent later drape folding as shale lateral to the lobe was compacted. Markers below the lobe do not exhibit this folding.
In another example, a productive east-trending Winters sand body is bordered on its north flank by a down-to-the-north normal fault that offsets younger units but is not evident below the Winters. The fault is apparently due to compaction of shale adjacent to the sand body, causing fracturing of more-brittle (Starkey Formation) sand bodies above and flowage of ductile shale below.