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
(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.