Mineralogical and geochemical characterization of the Miocene-Oligocene Santos Shale, Southern San Joaquin Valley, California

The Santos Shale is a 29 - 20 Ma member of the Temblor Formation located in the southwest corner of the San Joaquin Basin of California. It was deposited in an ocean basin along the slope and the basin plain. The Santos outcrops in the Temblor Range along the western perimeter of the San Joaquin Valley, where it dips and thins eastward into the subsurface under the valley. The goal of the study is to characterize the Santos Shale through its mineralogical and geochemical properties to better understand depositional environment and potential as a source rock or fractured shale reservoir. Forty samples were used for this study from the wells along the crest and the flank of the Belgian Anticline, ranging in depths of 1,061 feet in the west and 14,970 feet in the east. XRD analyses revealed that the bulk composition of the average Santos is composed of 45% clays, 29% quartz, 15% carbonates and 11% feldspar. In the clay size fraction, the clays are on average composed of 45.5% smectite, 43.3% illite, 8.2% kaolinite and 3.0% chlorite. LOI reveals the Santos has an overall high TOC content that ranges from 1.4 to 15.8 wt-%, with an average of 7.7 wt-%. Geochemical analyses from XRF and ICP-MS indicate the Santos was deposited in a well oxygenated marine environment based on relationships between iron, TOC and total sulfur and low trace element ratios used as paleoredox proxies such as Ni/Co vs. V(V+Ni), V/Sc vs. V(V+Ni), and V/Cr vs. Ni/Co. Redox conditions upon burial are indicated by enrichment in Ni, V and U within the Santos samples. High TOC content is attributed to the oxic ocean environment with productive paleo waters. The preservation of the organic matter is due to redox conditions upon burial at the ocean-sediment interface where the oxygen is quickly depleted from settling organic matter. High TOC values in the Santos imply very good potential for fracture development.