Lombardi reservoir, San Ardo field, California: Sequence stratigraphy in a deltaic environment associated with local tectoni

Abstract

The Lombardi reservoir at San Ardo oil field in California is extremely sand rich (>90%) and appears to be a relatively simple reservoir several hundred feet thick. The purpose of this study is to characterize stratigraphic complexities in the Lombardi reservoir that may affect later production and interpret how changes in stratigraphic architecture are tied to local syndepositional tectonism. Two parasequence sets are present in the Lombardi: the reservoir exhibits clinoform geometries along the western edge of the field and stacking patterns within the reservoir change across the field. Based on stacking pattern, core, and correlation studies, two possible sequence boundaries are recognized in the Lombardi sand. A lower sequence boundary is proposed approximately in the middle of the Lombardi sand, and an upper sequence boundary is proposed at the top of the Lombardi sand. In the southern part of the field, the Lombardi sand consists of two progradational parasequence sets, separated by the mid-Lombardi sequence boundary. The upper sequence boundary clearly truncates underlying folded Lombardi sand. Just a few miles (∼5 km) away, in the northwest part of the field, the Lombardi sand consists of a lower progradational parasequence set and an upper retrogradational parasequence set, separated by the mid-Lombardi sequence boundary. The upper sequence boundary at the top of the Lombardi sand shows little evidence of truncation of underlying units and is recognized by onlapping of younger strata. The difference in reservoir architecture between the south and northwest part of the field, a distance of only a few miles (∼5 km), is interpreted to be related to syndepositional uplift accompanying tectonism in the south. In core, the Lombardi sand consists mostly of homogeneous sand and bioturbated sand, arranged in upward-coarsening successions. The Lombardi sand is interpreted to be the product of deltaic deposition based on (1) the presence of homogeneous and laminated sandstones interpreted to represent mass flow deposits and (2) its restricted geographic distribution within the Salinas basin. No evidence of deposition in a wave-dominated or tidal-dominated shoreline is indicated. The interpretation presented here is that the sediments were deposited on the front of a delta emptying into a wave-protected estuary or bay, with minimal tidal effects. Complexities in stratigraphic architecture along the western edge of the field do not currently affect production but will likely become important during later stages of field development. That is, in the developed part of the field, reservoir continuity is not an issue, because of high sand content and sheet-like geometries. However, at the margins of the field, which will be produced in the future, clinoform geometries and sandstone tongues clearly affect reservoir continuity and will create vertical compartments during draining.