Petrology and Porosity Development of the Oligocene and Eocene Sandstones of the Wasco Oil Field, Central San Joaquin, California, USA

Olabisi, Olawale E.; Horton, Robert A.; Kaess, Alyssa B.; Caffee, Stephanie E.

The Wasco oil field is located in the southern San Joaquin Valley. Production was mainly from the Vedder Formation (Oligocene) and Eocene sandstones within the Kreyenhagen Formation. Depth of burial ranges from 3950-4275 m (Vedder) and 4575-4875 m (Eocene) and reservoir temperatures for both zones are ~135 C. The sandstones consist of fine- to coarse-grain very poor to well sorted angular to sub-round arkosic arenites. The Vedder sands have been interpreted as having been deposited in a shallow marine environment but this is based mostly on study of outcrops well to the east toward the basin margin; little is known about the Eocene depositional system except that the sands are enclosed within deep water shales. Porosity development in these deeply buried sandstones is mainly controlled by compaction and the dissolution of the framework grains with lesser influence due to cementation. The abundance of long grain-grain contacts, fractured grains, and broken framework grains is evidence that the sands are well compacted. Following deposition, mechanical compaction reduced primary porosity through the rotation of grains. Biotite and other labile grains deformed and occluded adjacent intergranular pores; fracturing of brittle grains and rotation of the pieces also reduced primary porosity. Feldspars are extensively albitized, biotite is commonly altered to chlorite, and volcanic-rock fragments exhibit extensive alteration to finely crystalline pseudomatrix. Dissolution of framework-grains created secondary porosity. Feldspars, volcanic rock fragments, quartz, biotite, and hornblende were all subjected to various degrees of dissolution. Framework-grain dissolution caused the formation of oversized and elongate pores resulting in secondary intergranular porosity. Continued compaction and precipitation of chlorite and laumontite cements reduced porosity. Thus, continuous compaction, precipitation of cement, and formation of pseudomatrix reduced primary porosity and possibly permeability. However, framework-grain dissolution created new pore spaces allowing maintenance of an open pore network, thereby facilitating the migration and accumulation of hydrocarbons in this reservoir.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013