Facies and sequence stratigraphic implications of the Statfjord formation (upper triassic-lower Jurassic), northern north sea, UK

Abstract

Detailed sedimentological logging of core, and interpretation of wireline logs from the Brent Oil Field, northern North Sea, have allowed for the identification of sedimentary facies and environments of the Upper Triassic-Lower Jurassic Statfjord Formation. The Statfjord Formation is interpreted to have been deposited within a diversity of sedimentary environments. These include delta plain, meandering rivers, braided rivers, tidally influenced fluvial systems and a shallow, high energy transgressive marine environment. Within the fluvially-dominated part, which is by far the most important volumetrically, two distinct facies associations were recognized: channel and overbank. The channel facies association is in turn divided into three discrete end members: low sinuosity, high sinuosity and minor/crevasse channel facies. Similarly, the overbank facies association comprises a variety of sedimentary facies including levee, crevasse splay, swamp, flood plain and rare lacustrine and lacustrine deltas. Low sinuosity channel facies are divided on the basis of their architectural style, into three different types. Type 1 low sinuosity channel facies dominates the northern part of the study area and resembles the sandy Platte River model of Miall (1977). Type 2 low sinuosity channel facies dominates the southern part of the study area and is characterized by numerous internal scour surfaces. This channel type is interpreted to have been deposited within a relatively proximal braided river system. Type 3 low sinuosity charmel facies is a transitional facies between the proceeding two low sinuosity channel types and is similar in character to the South Saskatchewan and the Battery Point River models of Cant and Walker (1978). Documented point bar sediments within the high sinuosity channel facies association indicate that channel bankfull depths may have reached as much as 4.5 m. The lack of evidence of exposure of the in-channel sediments in addition to the thick, consistent bedforms suggest that the rivers were probably of a perennial nature. Palaeoflow indicators obtained form HDT log analysis reveal that the Statfjord Formation m the Brent Field has been deposited by a north to northwest flowing drainage systems. Thorium/potassium cross-plots from NGS logs indicate an upward reduction in sandstone mineralogical maturity which may imply an upward reduction in the degree of reworking of sediments and a least sediment transport. The overall spatial and temporal consistency of the palaeocurrent indicators coupled with the upward decrease of sandstone maturity further indicate that no flow reversal occurred throughout the geological evolution of the Statfjord Formation. Sequence stratigraphic concepts applied in conjunction with detailed facies analysis indicate that sedimentary facies of the Statfjord Formation are arranged within two successive depositional sequences. Although the sequences are incomplete they possess all the attributes of the depositional sequences described by Posamentier and Vail (1988) and Van Wagoner et al. (1990). The lower sequence comprises sediments deposited within a highstand systems tract. The lowermost part is characterised by low net/gross ratio and is interpreted to have been deposited within early highstand systems tract most likely in an upper delta plain setting. As the rate of additional subaerial accommodation space approaches zero during a stillstand and eventually reverses, high sinuosity rivers are thought to have migrated laterally during a late highstand combing the previously deposited fine-grained sediments. The upper sequence overlies a regional sequence boundary unconformity and comprises an early lowstand wedge systems tract which is characterised by braided river sediments deposited during a slightiy rising base level. As the rate of additional subaerial accommodation space increases isolated high sinuosity river sediments are interpreted to have been deposited during a late lowstand wedge systems tract. Cyclic rhythmites reflecting neap-spring-neap tidal cyclicity of a diurnal palaeotidal regime represent the first 'local' major marine flooding. These are probably equivalent to a condensed section in the coeval marine realm and mark the onset of a transgressive systems tract: Continued rnarine transgression is documented by the deposition of the overlying high-energy, transgressive marine sandstone which is probably separated from the underlying tidally influenced strata by the occurrence of a ravinement surface indicating a progressively landward migrating shore line. In. considering the relative tectonic quiescence of the area during the deposition of the formation which occurred during a post-rift thermal subsidence phase, the close link between variations m the stacking patterns of facies tracts and the eustatic curve reveal that the stratigraphy and deposition of the Statfjord Formation were largely controlled by processes of sea level fluctuations in a near-shore setting.