Spatio-temporal Relationship between Upper Jurassic Mass Flow Sandstones and MTCs, Quad 30, UK Central North Sea
The influence of mass-transport complexes (MTCs) on the distribution of sandstone in deep water basins has been highlighted in recent research, e.g. studies of the passive margins of the Gulf of Mexico and offshore Brazil. Deep marine sandstones within the Upper Jurassic syn-rift succession of Quad 30, Central Graben, UK North Sea, are affiliated with large-scale MTCs. Three potential scenarios exist for the relationship between the MTCs and reservoir sandstones:
1. MTCs pre-dated
deposition of the sandstones, creating depositional topography, with potential
to influence the distribution of reservoir facies.
2. Contemporaneous deposition, where MTCs graded
distally into sandstones may imply connectivity between the sandstones and MTCs.
3. MTCs post-dated sandstone deposition and may have disrupted the reservoir interval.
Subsurface data has been integrated to identify proximal olistoliths and debris flows. Seismic indicates the MTCs were supplied by footwall collapse from major faults bordering the Jade and Judy horsts, which are composed of Triassic strata. Wireline log signatures identified in the olistoliths indicate a Triassic source. Core studies of the debris flows identified clasts of Triassic sediments. The MTCs are interbedded with marine mudstones and coarse sandstone. Distally massive, fine grained sandstones are implied to be the product of mass flows into the deep marine environment and form the reservoir interval.
Terrestrial palynomorphs recovered from mudstones surrounding the Jade and Judy structures demonstrate that the Judy Horst was subaerially exposed, with a variety of lowland ecosystems developed on the horst crest. The Jade structure did not form a significant island, as the mudstones lack a true ecological association of terrestrial vegetation. Variations in the ecologies on the Judy Horst may have been linked to changes in rift intensity.
Dinoflagellate cyst biostratigraphy has determined the temporal relationship of the MTCs and reservoir and has been used in production of a new chronostratigraphic framework for the region. Biostratigraphy demonstrates that MTCs were not contemporaneous with deposition of the sandstones. Deposition of the reservoir facies represented the initial rift stage, whilst MTCs were deposited during the rift climax. The evolution of the rift related topography and consequent level of horst exposure has implications for regional paleogeography, sediment pathways and provenance of the sandstone bodies.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California