Subsurface Insights | 9
typically associated with high primary reservoir quality . This is the result of karstification , which can occur during subaerial exposure of the platform top as a result of sea-level fall or tectonic uplift ( Hollis , 2011 ; Van Buchem et al ., 2011 ). Karstification on exposed platform tops could have occurred at sequence boundaries throughout the Albian – Turonian , but was particularly prevalent underlying sequence boundaries at the base of the Middle Cenomanian and beneath the regional Turonian unconformity ( Malekzadeh et al ., 2020 ). Reservoir quality beneath these karstified surfaces is significantly enhanced , which is often essential to achieve sufficient reservoir properties in platform interior environments .
Importantly , both of these controls on reservoir quality can be evaluated from Neftex GDE maps . Primary reservoir quality can be assessed based on paleogeography and the configuration of grainstone and bioherm depositional facies belts . Secondary reservoir quality related to karstification can be predicted within the sequence stratigraphic and tectonostratigraphic framework .
THE NEED FOR HIGHSTAND GROSS DEPOSITIONAL ENVIRONMENT MAPS
Neftex GDE maps are typically drawn at the point of maximum transgression ( MFS ) and the point of maximum regression ( LST ). However , rudist-rich carbonate reservoirs are typically associated with progradation within the HST prior to the exposure of the carbonate platform . In order to capture the spatial configuration of these rudist-rich facies belts , additional HST GDE maps have been published by Neftex for the Middle Cretaceous within our WebMapper application . The HST map complements the MFS and LST maps to capture the important petroleum system elements through the full sea-level cycle . In this stratigraphic interval , the framework enables the depiction of source ( MFS ), reservoir ( HST ), and reservoir enhancement ( LST ) within a single depositional sequence ( Figure 5 ).
EXPLORATION POTENTIAL IN SUBCROP TRAPS
Variable preservation beneath the erosional Turonian regional unconformity resulted in subcrop across the southern and central part of the Arabian Plate . There is high exploration potential for stratigraphic plays within subcrop traps where karstified rudist-rich carbonates are juxtaposed against the unconformity . A Coniacian claystone unit ( Laffan Formation and equivalents ) overlies this unconformity , forming an effective regional seal ( Al Zaabi et al ., 2010 ; Franco et al ., 2018 ).
By mapping the preservation limits on each Neftex GDE map surface , the maps were processed to give the subcrop extent within each sequence . The HST maps were used to assess the extent of potential reservoir facies beneath the subcrop , based on prediction of high primary reservoir quality ( grainstone
Figure 5 > Simplified MFS , HST , and LST Neftex ® Gross Depositional Environment ( GDE ) maps for a single Cenomanian depositional sequence across the southern part of the Arabian Plate . Note the rudist-rich carbonate belt depicted on the HST GDE map ; this facies belt has high reservoir potential , particularly if karstified on the subsequent LST .