Deep-Water Gravity Flow Deposits of the Onshore Eel River Basin Rio Dell Formation: Log Character and Net-Sand Calibration
The Rio Dell Formation of northwestern California’s onshore Eel River basin records generally shoaling basinal conditions throughout the late Pliocene. The lower Rio Dell Formation (LRD) is comprised of hemi-pelagic mudstones, as well as interbedded muddy siltstones and fine- to very fine-grained sandstones. These rocks were initially deposited in upper middle bathyal to upper bathyal depths. The interbedded muddy siltstones and sandstones of the LRD form several sand packages, each approximately 100 feet thick. These sand packages are interpreted to be a succession of “linked-debrite” gravity flow deposits.
Several wells at Grizzly Bluff gas field in the onshore Eel River basin have open-hole well logs through the entire Rio Dell Formation. Within the LRD sand packages, highest quality reservoir sandstones have salient SP responses and resistivity values as high as 7 ohmm. Their log signatures have a serrated character, and blocky, channel-type signatures are very rare. Calibrations of standard well logs to fully described LRD conventional cores and outcrop sections provide for more accurate net-sand tabulations, and reveal that discrete sandstone sub-units less than 2 feet thick cannot be fully resolved on these logs. Thickest sandstone sub-units – often only 2 feet thick – have highest permeability and porosity values. Hence, standard well logs alone do not provide adequate tools for a full appraisal of N:G ratios within these thin-bedded sand packages. If image logs are not available on a well-by-well basis, then calibration to at least one representative conventional core becomes critical for evaluating reservoir quality and net-sand thickness.
AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009