Core Description, Lithofacies Interpretation and Sedimentological Characterization of the Pleistocene Tulare Formation - Mixed Provenance Alluvial Fan, Midway Sunset Field, Kern County, California

Fisher, Emily
[email protected]

The Pleistocene Tulare Formation in the Midway Sunset Field (MWSS), San Joaquin Basin, records a unique and complex mixed facies of alluvial, braided fluvial and lacustrine environments. Due to continuous tectonic activity, changes in depositional environment can be rapid both laterally and temporally. This study describes a shallow, undocumented section of the Tulare Formation found in western MWSS called the Upper Tulare.

Two hundred feet of continuous core was obtained in the Upper Tulare section MWSS. The core was described and categorized into seven lithofacies: (1) orange-brown massive matrix-supported conglomerate with silt and fine-grained sand and clay framework grains. (2) light-brown massive matrix-supported conglomerate with siltstone matrix and angular to sub-rounded granules and pebbles. (3) massive coarse-grained conglomerate with framework grains of sand and high volumes of rounded diatomite clasts. (4) alternating wavy beds and lamina .5-2 cm thick of interbedded clays and fine-grained sand and occasional pebble beds, characterized by diffuse contacts and soft-sediment deformation. (5) normally graded fine- to course- grained sandstones 30 cm thick punctuated by beds of granule to pebbles and 2 mm mud drapes and clay beds. (6) massive, course-grained well-rounded unconsolidated quartz sand. (7) laminated blue-gray and white claystone with wavy deformation.

Core plugs were sampled every foot, while ultraviolet light photography were taken and the samples analyzed for porosity, permeability and saturation measurements. Two samples were identified for further analysis by thin-section petrography, Scanning Electron Microscopy (SEM), X-ray diffraction (XRD). Five samples were selected for viscosity, API gravity, Saturate-Aromatic-Resin-Asphaltene geochemical assessment and pyrochromotography.

Seventy percent bulk volume of the cored interval is matrix-supported conglomerates. In each conglomerate facies, the clasts are primarily tan and white sub-angular to sub-rounded, Opal-A and Opal-CT diatomite, .2-1.5 cm in diameter. Rounded plutonic, metamorphic, volcanic and limestone clasts are also present. Thin section and SEM of the framework grains in these conglomerates display heterogeneity at 1mm scale; accessory grains include glauconite, phosphatic fragments, muscovite, amphibole, biotie, and organic particles. Quartz, feldspars, and clay detected by X-ray diffraction occur principally in the mudstone matrix whereas the two forms of opal are found in the porcelanite clasts. The porcelanite clasts exhibit moldic pores and matrix micropores. The clasts are composed of siliceous microfossils, primarily opal-A, with some areas illustrating partial conversion to opal-CT lepispheres. The presence of both opal-A and opal-CT diatomite silica phases is confirmed by XRD and SEM.

In the Upper Tulare, the low density and clay-rich sediment sources erode and deposit unusually fine-grained alluvial fan sequence while sedimentary structures reflect a high energy environment. Structures include dish and flame structures, wavy lamina, and convoluted bedding. Within the sandstone facies, pore spaces between silica and plagioclase grains contain an average of 15% intergranular clays. The sandstones are microporous due to the presence of detrital smecitite, Illite and mica in some primary pores.

The Upper Tulare is of an alluvial fan depositional environment which has clasts sourced from both Miocene diatomaceous Monterey and Miocene-Oligocene siliciclastic Temblor Formations. The Temblor Formation is a marine clastic reservoir composed of arkosic sandstone and micaceous claystone. The Monterey is composted of deep marine diatomite and siliceous shale. The two formations were synchronously uplifted and eroded due to transpression along the San Andreas Fault. Sediments were deposited north-eastward into the San Joaquin Valley basin.

The heterogeneous sediment and the poor sorting result in a high porosity (35%), low permeability (3-100 md) reservoir which contains 30-60% oil saturation of heavy (12 API gravity) biodegraded oil. Good reservoir quality is isolated in the sandstones and diatomite clast by the pervasive pore-filling clays. With heavy oil a greater permeability would be required to properly steam flood this alluvial fan facies.

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