Nonmarine sequence-stratigraphic concepts and application to reservoir description in the Statfjord Formation, Statfjord Field, northern North Sea

Abstract

Nonmarine sequence-stratigraphic concepts were applied in a detailed analysis of the Statfjord Formation of the Statfjord Field. Five main depositional facies were recognized in the formation based on extensive core studies. The primary reservoir facies are the multistory channel sandstones which were deposited by braided-stream channels in broad paleovalleys cut during base-level drops by knickpoint migration. Deposition of this reservoir occurred during the base-level drop in the early lowstand and initial base-level rise in the late lowstand. The multistory sandstones consist of stacked channel remnants that each fine upward and have truncational bases. The remnants range in thickness from 0.1 to 4 m. The lateral continuity of the multistory channel sandstones is good. Finer-grained flood-plain deposits sharply overlie the multistory sandstones. The finer-grained deposits consist of crevasse splays, point bars, levees, and thick mudstones of possible lacustrine origin. Soil horizons are a common component of the mudstones. Deposition of these finer-grained deposits occurred in the transgressive and highstand parts of the sequence. The truncational bases of the multistory channel sandstones are defined as potential sequence boundaries. They represent widespread erosional surfaces interpreted to have formed during a regional fall in base level. Seventeen potential sequence boundaries were identified and correlated in the Statfjord Formation. They provide a chronostratigraphic framework for predicting lithology and reservoir facies and represent approximate time lines within the formation. The sequence-stratigraphic correlations show significant intercutting of the sandstone bodies in the Upper Statfjord Formation and also truncation of the floodplain deposits by the sequence boundaries. In the Lower Statfjord Formation, however, there is not as much intercutting between the braided-stream sandstones, and consequently the shale intervals are laterally more continuous. Hence, vertical reservoir communication is more restricted in the Lower than in the Upper Statfjord Formation. The sequence-stratigraphic concepts proved to be a useful tool to develop a reservoir description for this complex, nonmarine formation and provided a model to aid in the evaluation of the lateral continuity of the reservoir sandstones as well as the vertical communication between them.