Subsurface Insights | 7
These outcrop sections were incorporated into regional stratigraphic transects that characterize the morphological and depositional evolution of the Middle Cretaceous carbonate system . The understanding from these transects was used to generate a conceptual model ( Figure 2 ) for this depositional system during a Neftex 3 rd order sequence :
1 . The model starts at the sequence boundary ( SB ) with a carbonate ramp with a flat depositional profile . In the early transgressive systems tract ( TST ), retrogradation of claystones across the ramp occurred before aggregational carbonates re-established .
2 . In the late TST , the rate of sea-level rise accelerated . In intrashelf basin areas , the carbonate system was unable to keep pace with sea-level rise and was drowned . However , the carbonate system within the platform interior continued to aggrade throughout the TST . This differential aggradation is what created the intrashelf basin morphology under tectonic quintessence . Restriction within the intrashelf basins resulted in bottom-water anoxia and the deposition of organic-rich carbonates .
3 . In the highstand ( HST ), progradation of the carbonate margin occurred as carbonate production exceeded accommodation . Grainy carbonate shoals with rudist-rich grainstones and rudstones ( high primary reservoir quality ) were characteristic of the HST .
4 . In long-lived , multi-sequence intrashelf basins , such as the Shilaif ( UAE ) and the Najaf ( Iraq ), there was remaining accommodation during the lowstand ( LST ). In these instances , the LST was typically suppressed , as exposure of the platform dramatically reduced the aerial extent of carbonate productivity and the amount of shedding into the basin . The LST wedge was aerially limited and basinal LST deposits were often condensed .
UNDERSTANDING THE DISTRIBUTION OF PETROLEUM SYSTEM ELEMENTS
Regional correlations are invaluable in understanding the distribution of source , reservoir , and seal . These petroleum system elements are highlighted by a regional correlation ( Figure 3 ) that extends over 500 km from Oman , through the Rub Al Khali area of Saudi Arabia , to the margin of the Shilaif intrashelf basin in the United Arab Emirates . This correlation is flattened at the Albian – Cenomanian boundary , which is a surface with a relatively flat depositional profile . This datum is useful as it reveals the depositional morphology of the Cenomanian – Turonian interval .
The correlation highlights the consistent expression of the depositional sequences between Oman and Saudi Arabia along depositional strike . Eustacy-controlled , intrashelf basin development occurred in the Early Cenomanian and Late Cenomanian sequences , with the deposition of organic-rich carbonates that are potential source rocks where thermally mature . In contrast , a muddy ramp system developed in the Middle Cenomanian . This interval contains retrogradational siliciclastic units , which could form intraformational seals . Some
Figure 3 > Regional correlation indicating the distribution of petroleum system elements within a sequence stratigraphic , paleogeographic , and morphological framework . The Al (%) log is a proxy for siliciclastic content , while the Mo ( ppm ) log is a proxy for anoxia . Well data are derived from van Buchem , ( 2002 ), Homewood et al ., ( 2008 ), Craigie , ( 2015 ), and Perrotta , ( 2017 ), and are available within the Neftex ® Wells and Outcrops Database .