Heterogeneous Oil Saturation in Submarine Channel and Adjacent Facies, Monterey Formation, Point Fermin, Palos Verdes, California

Al Shammary, Nawaf and Behl, Richard J.
[email protected]

Extreme heterogeneity in oil saturation between closely adjacent sandstone beds reflect different timing and degree of diagenesis, apparently due to local sedimentary and geochemical environment within the Miocene Monterey Formation. Understanding the distribution and origin of such heterogeneity is critical to effectively exploiting intercalated sandstone deposits within fine-grained unconventional reservoirs, such as siliceous or organic-rich shales and in unraveling subtleties of stratigraphic traps. Sea cliff exposures at Point Fermin, California, expose a submarine channel facies – the Point Fermin Sandstone member – within the largely hemipelagic facies of the organic-rich and siliceous Altamira Shale member. Separated by only meters, sandstone beds in the former are oil-saturated, whereas sandstone beds in the latter are not. To understand this, we evaluated the role of provenance, diagenesis, and sedimentary texture on the disparate saturation. 32 samples from both units were analyzed for porosity, permeability and fluid saturation in conjunction with thin-section petrographic analysis. Thin sections were point counted by a modified Gazzi-Dickinson method to determine variation in provenance and to establish the types and degree of porosity and cementation. Sandstones are primarily schist-bearing lithic arenites with substantial metamorphic and sedimentary rock fragments. Both inter- and some intragranular porosity exist in these samples and grains are cemented mosly by euhedral-subhedral, rhombic dolomite. Initial data show that both units have the same provenance but differ in the timing and type of diagenesis with shale-hosted sandstones generally showing earlier cementation. Early cementation occluded pore spaces prior to hydrocarbon migration and charging of the sandstone. Sandstone bodies within the two members show different packing, grain-to-grain contacts, and intergranular volume. Within the Point Fermin Sandstone, grain size and sorting is also important and the degree of cementation varies by position in the stratigraphic architecture, although understanding this distribution needs further, focused study.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013