Dissolution and Porosity Development in Uncemented Miocene Reservoir Sandstones, Western San Joaquin Basin, California

Lower and middle Miocene sandstones of the western San Joaquin basin consist of feldspathic and volcaniclastic sands deposited in a tectonically active area. These rocks have been subjected to repeated uplift and subsidence and have complex diagenetic histories which vary from field to field, but similar trends in porosity development have been observed in several west-side oil fields. Following deposition, carbonate cements may or may not have precipitated. Uncemented sands underwent significant compaction due to burial and possible tectonic stress as well. This resulted in crushing and deformation of grains, including quartz, and in modification of grain boundaries at grain-grain contacts. alteration of unstable grains, especially those of volcanic origin, resulted in production of clays which were often squeezed into interstitial pores. In many cases, the net effect of alteration and compaction was destruction of most primary porosity. This was followed by a period of extensive grain dissolution, especially of feldspars, resulting in significant secondary porosity. Continued compaction modified many secondary pores resulting in intergranular secondary pores which resemble primary pores. In many examples studies, as much as 80% or more of the intergranular pores probably formed in this manner. Recognition of this process has important implications in terms ofmore » fluid flow and mass transport, as well as initial sediment composition and provenance.« less