Datapages, Inc.Print this page

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

The Upper Triassic Minjur Formation: A New Bio- and Sequence Stratigraphic Framework and Its Implications for Hydrocarbon Prospectivity

Neil S. Jones1; Nigel P. Hooker2

(1) Exploration Resource Assessment, Saudi Aramco, Ras Tanura, Saudi Arabia.

(2) Exploration Technical Services, Saudi Aramco, Dhahran, Saudi Arabia.

The Upper Triassic (Norian to Rhaetian) Minjur Formation is a siliciclastic succession present at surface and also in the subsurface across large parts of central and southern Saudi Arabia. Until recently its stratigraphy and sedimentology have been poorly understood and it has largely been ignored as a target for hydrocarbon exploration. A recent study by the Area Exploration Department of Saudi Aramco built a regional bio- and sequence stratigraphic framework to support regional exploration efforts in Central Arabia and better understand its prospectivity. This study utilizes core and cuttings data from 26 wells, combined with wireline logs from an additional 100 wells. It should be noted that all 600 ft of available Minjur Formation core in Saudi Arabia was studied.

The core-based sedimentology, together with palynology from core and cuttings samples, indicates palaeoenvironments that include extensive coastal alluvial plain, with fluvial channels and adjacent floodplain environments, passing basinwards into shoreline facies, typically marked by tidal, deltaic and marine settings. Routine core analysis data indicates that the best potential reservoirs are the fluvial channel facies, with porosities of 15 to 25% and wide permeability range from 100 md up to 10 darcies.

Wireline log correlations were integrated with new biostratigraphic data to establish five depositional sequences, from the base of the Minjur up to the unconformity at the base of the overlying Marrat Formation (the Pre-Marrat Unconformity). Lowstand, transgressive and highstand systems tracts have been identified from gamma log character and show distinctive proximal to distal changes. Only one palynofloral unit of a non-marine character (T1 Palynozone) was previously recognized within the Minjur Formation, but new palynological data and interpretations have established a four-fold biostratigraphic subdivision. Palynofacies provide evidence of marine to non-marine palaeoenvironmental variations within each log-defined sequence.

The new stratigraphic framework is important because it predicts the position and systems tract of the main reservoir and source rock facies. It further allows better risk assessment of various existing play types, and has helped identify new play concepts in the previously under explored Minjur Formation.