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AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

How Deepwater Systems Organise at Multiple Scales

Stephen Flint1; David Hodgson1; Anthony Sprague2; Rufus L. Brunt1; Jorge Figueiredo1; Darren Box2; Willem Van der Merwe1; Claudio Di Celma3; Amandine Prelat1

(1) University of Liverpool, Liverpool, United Kingdom.

(2) Exxonmobil Upstream Research Company, Houston, TX.

(3) University of Camerino, Camerino, Italy.

Deepwater clastic systems show a similar tripartite organization at both composite sequence and composite sequence set scales, allowing prediction of geometries, net:gross distribution and reservoir connectivity at seismic and sub-seismic scales. Over 10 years of outcrop studies from the Karoo basin have mapped systems tracts, sequences and composite sequences over more than 500 km2. Typically, sequences include a 5-60 m thick sandstone-dominated lowstand systems tract overlain by a 1-5 m thick hemipelagic claystone of regional extent that is interpreted as the TST/HST shutdown of the deepwater system. Sequences stack into composite sequences that include a lowstand sequence set of sandstone dominated sequences overlain by a regional mudstone-dominated succession of siltstones and hemipelagic claystones, interpreted as the transgressive/highstand sequence set. In both basin floor distributive systems and slope channel-levee systems, each composite sequence usually comprises three sequences. In slope deposits, the lowstand systems tracts of the three sequences include a basal LST dominated by distributive lobe deposits, a middle sequence in which the LST is dominated by channel-levee complexes and an upper sequence that shows either deeply entrenched slope valley/canyon deposits or a backstepping distributive system. Complexities occur when slope valleys/canyons of a younger sequence cut right through older sequences. The 320 m thick basin floor fan succession comprises three composite sequences that show a progradational-aggradational-retrogradational stacking pattern, forming a composite sequence set. The 1 km thick slope succession comprises two composite sequence sets, both of which comprise three composite sequences. These show similar characteristics to the sequences: a lower composite sequence set of largely distributive deposits, a middle CSS dominated by channel-levee complexes and an upper CSS that includes incisional slope valleys/canyons.

Within basin floor distributive systems, net:gross distribution is highest in the aggradational middle composite sequence. Slope successions are more complicated due to across-strike variability and variable amounts of incision but the lower composite sequence is sandstone-dominated, the middle composite sequence has high net channel-fills and low net:gross external levees and entrenched slope valleys/canyons of the upper composite sequence are variable to low net.