Tectono-eustatic controls on reservoir compartmentalisation and quality — an example from the Upper Miocene of the San Joaquin basin, California

Abstract

Systematic changes in stratigraphic architecture and reservoir quality of Upper Miocene Stevens sandstones at North and South Coles Levee fields, San Joaquin basin reflect tectono-eustatic controls on deposition. Two depositional sequences (330–500 m thick) are present in the Coles Levee Stevens. The upper sequence contains sandstones deposited by sediment-gravity flows in water depths of 300–500 m. Backstepping (retrogradational) reservoir compartments in this sequence overlie vertically stacked to prograding compartments. Although the backsteps are the better reservoirs with higher permeabilities and porosities, the vertically stacked to prograding compartments are volumetrically more important. Curiously, the upper Stevens section is sand-rich (60–80% ss) on structural highs that were also palaeotopographic highs. Thicker sections with less and characterise the flanks. These sand distributions are anomalous since sediment-gravity flows preferentially deposit in bathymetric lows. Apparently, the flanks of Stevens palaeohighs subsided faster than the crests. Consequently, accomodation increases were greatest on the flanks and resulted in aggradation with preservation of section. Less accommodation on the crests resulted in progradation with erosion and reworking. Through lobe-switching processes some sand reached all parts of the depocenter. Subsequent progradation with erosion of shale interbeds and attendant reworking by density currents created sand-rich sections in slowly subsiding areas (crests). Concurrent aggradation preserved shale interbeds and created shale-dominated sections in rapidly subsiding areas (flanks). Greater compaction of thicker, shale-dominated flanks equalised thicknesses of flank and crest sections. Thus, the reservoir architecture represents the integrated effects of compensation (lobe-switching), accommodation (subsidence + sea level), and compaction.