Structural Evolution Impact on Reservoir Quality Distribution in an Early Cretaceous Carbonate Reservoir, UAE
Authors: McGeer Alicia , Sellar Christopher , Verma Ravi , Al Naqbi Dhabia 
Published: November 2020

The structural evolution and timing of hydrocarbon charge potentially controls the style of diagenetic overprint and the consequent reservoir quality distribution in "Oilfield A." Tilting due to regional tectonic events may have repositioned fluid contacts and influenced the development of stylolites, cementation and microporosity development. A reconstruction of the palaeo structure of Oilfield A was undertaken in order to identify the key structural and diagenetic events, constrain their timing and tie them to seismic properties.

Well picks, interpreted 3D seismic horizons and P-Impedance from the inversion of PSTM 3D seismic data were used in this study. The main reservoir from the Early Cretaceous was reconstructed by flattening on progressively shallower overburden horizons. Cross-sections dissecting the structure help to identify structural events. Published data on diagenetic events were reviewed from analogue oilfields and compared to the timing and burial depth of Reservoir 2 in Oilfield A. All key regional structural events and regional petroleum systems evolution were reconciled against the burial history of Oilfield-A. Each structural time step is compared to seismic inversion property P-impedance.

Structural flattening of the Reservoir 2 seismic horizon using successively shallower overburden surfaces reveals that a structural four-way dip closure has existed in Oilfield A since the Late Cretaceous. The main closure was initially located in the north-east of the present day field with the deeper flank located to the south-west. Hydrocarbon maturity and migration, potentially from both Jurassic and Early Cretaceous sources, began in the Late Cretaceous and continued through the Tertiary. The structure was tilted towards the north during the Oligo-Miocene Zagros orogeny. At this time, the crest of the field appears to have migrated towards the north-west and the south of the field was uplifted. The palaeo free water level is likely to have been driven deeper in the north and oil may have migrated south into areas of the field previously beneath the palaeo free water level. A relationship is suggested between the position of the palaeostructural crest and low values of P-impedance from seismic inversion (P-impedance is negatively correlated to porosity). This mapping exercise supports the geological concept that oil charge was sufficiently early to have potentially prevented significant cementation on the crest of the field while flank areas became chemically compacted during burial and are consequently more heavily cemented.

Structural evolution and hydrocarbon charge are rarely considered as key diagenetic inputs affecting reservoir quality distribution in carbonate reservoirs. Loosely constrained paragenetic sequences are common in reservoir characterization studies. This study uses exploration-style structural reconstruction techniques on a carbonate oilfield development. Identification of key tectonic events and detailed understanding of the timing of hydrocarbon migration into a reservoir are fundamental prior to developing a geological concept and undertaking detailed subsurface modeling of reservoir properties.