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AAPG GEO 2010 Middle East
Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain

Multi-Scale Assessment of the Middle Eastern Permo-Triassic Khuff Carbonate: Structural Evolution and Its Impact from Reservoir Properties

Valentina Zampetti1; Ravi Borkhataria2; Marietta Vroon2

(1) QSRTC, Shell, Doha, Qatar.

(2) 2Shell International Exploration and Production, Rijswijk, Netherlands.

The Khuff Petroleum System Study is a multi-scale, multi-disciplinary analysis that integrates subsurface and outcrop, rock and fluid samples, and static and dynamic data in order to characterize the Permo-Triassic Khuff carbonate, one of the major petroleum reservoirs in the Middle East region.

At regional scale, the Khuff carbonate shows a variety of depositional environments (with facies ranging from coastal plain anhydritic claystone, tidal flat/low-to-high energy lagoonal deposits to open-marine dolostones alternating with grainy limestones and high-energy shoal-dominated dolostone/thick grainy limestones) and thicknesses (from near zero at the pinch-out of siliciclastic facies in central Saudi Arabia, to more than 400 m (1300 ft) in Ghawar field in northern Saudi Arabia, expanding to 800 m (2600 ft) in the North field, Qatar and to nearly 1000 m (3300 ft) in the eastern United Arab Emirates).

Local seismic data calibrated to regional well correlations indicate that Khuff-thickness, lithology and facies distributions are strongly controlled by the inherited structural relief e.g. by the reactivation of the basement structural fabric related to Permian-Triassic tectonic events.
Following the regional tectonic setting, the study area can be subdivided in 4 mega-structural provinces: (i) the Arabian Peninsula, (ii) Oman, (iii) the Zagros region and (iv) the salt provinces.

At the location of the Arabian Peninsula, Pre-Khuff basement-related anisotropies are interpreted to have formed as early as Late Neo-Proterozoic to Ediacarian time and to follow four main trends: (i) N-S (Nabitah), (ii) NW-SE to WNW-ESE (Najd), (iii) NNW-SSE to NW-SE (Mesopotamian).
At the regional-scale the Lower Khuff thickness shows a step-wise increase from fault block to fault block towards the NE. This trend is consistent with an extensional/transtensional reactivation of the NW-SE to WNW-ESE pre-existing basement anisotropies. During the Middle to Upper Permian, the break-up of the Cimmerian terranes was associated with an azimuth of extension oriented around NNE-SSW implying the Lower Khuff to represent a syn-rift sequence. During the Early-Middle Triassic, the Cimmerian blocks moved rapidly away from the Arabian margin, opening up the NeoTethys behind them. The evolving active spreading centre exerted a ridge-push progressively stabilizing the passive margins of the opening oceanic basin. Accordingly, the stress field was re-oriented with a NE-SW-trending maximum horizontal stress direction allowing reactivation of the NS and the NNW-SSE/NW-SE trending basement fabrics. The resulting pattern of fault-interferences is interpreted to be responsible for the irregular thickness distribution observed in the Upper Khuff Member.

Within the mega-structural salt province, the producing fields and discoveries in the Southern Gulf Salt Basin show a surprising variability in depth, thickness, bulk lithology, average porosity, reservoir fluids and productivity. At a regional scale, the variability in reservoir properties identified in this study can be partially explained by different tectonic events such as fault reactivations. Also salt diapir-specific growth history coupled with large-scale burial alteration has played a role combined with deep burial in-situ modification of hydrocarbons in-place and displacement of hydrocarbons by late non-hydrocarbon charge.

In conclusion understanding the interaction between regional tectonic events, basement lineaments and sedimentation is a crucial and critical step in order to map paleogeography, thickness distributions, facies patterns and play fairways across the entire Arabian Plate.