Abstract: Tectonically Controlled Diagenesis of Immature Sandstone in the Western San Joaquin Basin, California

HORTON, ROBERT A., JR.

Lithic and feldspathic arenites of the Temblor Formation (Lower Miocene) are important reservoir sands in the western San Joaquin basin, including Coalinga and Jacalitos oil fields. Sands were derived from source areas to the west and deposited in deltaic to shallow marine environments on the margin of a rapidly subsiding basin. Tectonic activity associated with the San Andreas fault has caused repeated uplift along the western margin of the basin and allowed influx of meteoric water into the subsurface. Tectonically enhanced compaction resulted in severe fracturing of framework grains.

Diagenesis is dominated by alteration and squashing of volcanic lithic grains and fracturing, alteration, and dissolution of quartz and feldspar grains. Fracture-controlled albitization of plagioclase and potassium feldspars is common in some fields but absent in others. Albitization is temperature controlled and indicates diagenesis in marine derived pore fluids, in some cases at depths greater than present burial. Less common is fracture controlled alteration to phosphate. Cementation is minor. Pore-filling kaolinite is rarely present. Thin zones of calcite and/or dolomite occur as pore-occluding cement and as replacement of feldspars and quartz. Petrography, major-element chemistry, and isotopic signatures indicate a complex history of carbonate precipitation, dissolution, and replacement under changing geochemical conditions. Pseudomatrix formation in sands with a high percentage of labile grains cushioned the rocks from tectonic fracturing and reduced intergranular permeability, hereby limiting the effects of framework-grain dissolution. Where fewer labile grains were present, fracturing caused enhanced intergranular permeability and exposed fresh surfaces to diagenetic fluids resulting in extensive leaching of all grain types and significant changes in QFL ratios. The observed volume of authigenic clays is vastly insufficient to account for the material removed by dissolution suggesting that meteoric fluids were responsible for dissolving and transporting this material out of the system. Thus, tectonic controls on fluid movement and chemistry controlled diagenetic reactions.

Search and Discovery Article #90945©1997 AAPG Pacific Section Meeting, Bakersfield, California