Sedimentary Characteristics And Genesis Mechanisms Of The Middle Jurassic Brent Group Sandstone In The Viking Graben Of Northern North Sea

The Middle Jurassic Brent Group in the northern North Sea is believed to be good reservoirs. To get a proper understanding of the sedimentary characteristics, distribution,and origin of the Brent deposits, particularly these tidal reservoirs, this study investigated various aspects of the Brent Group of the Viking Graben, including pelaeogrography and tectonic movement, and re-examined the Brent Group deposits via core and log data. The detailed interpretation of core and logs and comprehensive analysis of depositional systems are based on the principle of sedimentology and sequence stratigraphy. Modern analogue systems are used to compare with the Brent deposits. The results show that the documented Brent Group succession can be divided into five 3rd-order sequence,including SQ2 A, SQ2 B, SQ2 C, SQ3 and SQ4. Within the sequence stratigraphy, a range of storm- and tide- dominated deposits as well as mixed energy deposits are welldeveloped in the Brent Group, including wave-dominated delta, wave-dominated shoreface, storm-dominated, tide-influenced shoreface, and tide-dominated estuary. The evolution of the Brent deposits within the sequence stratigraphic framework suggest that the Brent Group experienced frequent base-level rise and fall. The Brent Group was transited from a wave-dominated deltaic system upwards into a mixed storm-tide influenced shoreline system, due to the shoreline irregulation in upper Rannoch formation.This shoreline irregulation was amplified in the Etive- basal Ness formations, and led to the formation of embayed shape as well as the tide-dominated estuary. In the mid-upper Ness Formation, the Brent Delta was recovered and prograded northward, and followed by large-scale transgression during the Tarbert formation.In this study, we detailedly described the storm-dominated, tide-influenced shoreface and tide-dominated estuary deposits:(1) various types of storm-tide couplets were recognized storm-dominated, tide-influenced shoreface, the formation and preservation conditions of which indicate that there was locally frequent storm-event, high sediment supply, and strong tidal current, as well as locally high subsidence rates;(2) three types of bars were identified within the tide-dominated estuary, including tide-domianted,storm-tide influenced bar, tide-dominated, fluvial-infleunced, and tidal bar withoutsignificant wave and fluvial influence. These extremely thick tidal bars share common feature with the bar-complex that found in the Cobequid Bay, Bay of Fundy, Canada by showing alternation of progradational and retrogradational stacking pattern, implying frequent base level fall and rise. Theoretical model, aiming to interpret the origin of the tidal deposits and their evolution, suggests tidal deposits were controlled by local rather than large-scale palaeogeography and tectonic movement, the latter of which caused the irregulation of shoreline, amplification of tides, frequent base-level rise and fall, thus further leading to the formation and alternation of progradational and retrogradational stacking pattern of tidal bars. In addition, high subsidence rate was contributed to create large accommodation space and accumulation of thick tidal deposits. Comprehensive analysis of the the sequence stratigraphy and effective source-reservoir-cap assemblages,suggest that the main source rock was developed in the lower part of the SQ2A-RST sequence, most of the reservoirs were formed within the upper part of the SQ2A-RST,SQ2 B, SQ2 C and SQ3, and the major cap rock was developed in the SQ4.