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.

Oilfield A is a low relief structure consisting of multiple stacked reservoirs. Three reservoir intervals show different trends of reservoir property distribution. Lateral reservoir property degradation in Oilfield A is correlated to increased vertical heterogeneity. Understanding the driving mechanism for the controls on reservoir property distribution is therefore essential for effective well placement, well spacing and achieving the expected ultimate recovery from the oilfield. Previous integrated reservoir characterization studies in Oilfield A have identified a link between chemical compaction, reservoir thickness, cementation and reservoir property distribution in Reservoir 2. As part of reservoir model building and subsequent structural updates, thickness maps have been constructed from well data for each reservoir unit as an early indicator of possible reservoir property trends. The maps show trends which are integrated with other data, both static and dynamic in order to validate their impact. To establish correlations, thickness maps were compared to maps of log porosity, facies, diagenetic features, geophysical attributes, cumulative oil production and water cut. Reservoir thickness shows a good correlation to reservoir property distribution in several different reservoirs in Oilfield A. Reservoir 3 shows a south-east to north-west trend of reservoir degradation, Reservoir 2 shows a north to south trend of reservoir degradation, while Reservoir 1, shows a west to east trend of reservoir degradation. Chemical compaction has been shown to drive reservoir property degradation in all three reservoirs. An increased abundance of stylolites is noted in the thinnest, most cemented, poorest quality parts of the three reservoirs. Since chemical compaction (stylolitisation) is a process which starts in the burial diagenetic realm and since all three reservoirs show a similar paragenetic history it is logical to expect all three reservoirs have a similar burial history. However, the trends observed in the three stacked reservoir intervals are orthogonal to opposite. Reservoir properties are rarely randomly distributed in oil reservoirs. Integrating static and dynamic data together in Oilfield A has shown that compaction is the dominant driver of reservoir degradation in three stacked reservoirs. Understanding the trends controlling lateral and vertical reservoir heterogeneity due to compaction is essential in appraisal and development well planning as well as increasing expected ultimate recovery as part of increased and enhanced oil recovery projects.