THE INFLUENCE OF TECTONICS ON THE ENTRAPMENT OF OIL IN THE DEZFUL EMBAYMENT, ZAGROS FOLDBELT, IRAN.
M. L. Bordenave+* and J. A. Hegre **
Consultant, 24, avenue President Kennedy, 75016 Paris, France.
+ Author for correspondence email: Max.Bordenave@wanadoo.fr
**TOTAL, Cedex 47, 92069 Paris La Défense, France.
The timing of the orogenic events associated with the closure of South Tethys significantly influenced the generation, migration and entrapment of petroleum in the Zagros Foldbelt of Iran. This influence was particularly important in the Dezful Embayment, which is one of the world's richest oil provinces, containing some 8% of global oil reserves in an area of only 60,000 sq. km.
In the Dezful Embayment, oil and associated gas occurs in two carbonate reservoirs -- the Sarvak Formation of Cenomanian to Turonian age, and the Early Miocene Asmari Formation, sealed by the evaporites of the Gachsaran Formation. The oil and associated gas are trapped in large “whaleback” anticlines which resulted from the Neogene Zagros orogeny. Two excellent source rocks, the Albian Kazhdumi Formation and the upper part of the Pabdeh Formation (Middle Eocene to Early Oligocene), supplied the Asmari and Sarvak reservoirs and with them form the Middle Cretaceous to Early Miocene Petroleum System. This is independent of older petroleum systems.
Two particular problems are addressed in this paper:
The first is the relative timing of trap formation versus oil expulsion from the source rocks. If oil expulsion occurred prior to Zagros folding, the oil would have migrated along gently dipping ramps towards the Persian (Arabian) Gulf and Southern Iraq, and would have been trapped a long way from the source kitchen. By contrast, if oil expulsion took place when the whaleback anticlines already existed or had at least begun to develop, the oil generated would have moved almost vertically towards the nearest anticline.
Secondly we assess the type of heatflow to be used for modelling. This could be either variable or constant, depending on the stability or instability of the Arabian Platform and on subsidence variations during source rock maturation.
Our conclusions can be summarized as follows:
Firstly, we propose that the paroxysmal phase of Zagros folding commenced in the Dezful Embayment towards the end of the Middle Miocene around 10 Ma ago and continued throughout the Late Miocene and Pliocene.
Secondly, in the light of the remarkable stability of the Arabian Platform for some 260 Ma, with almost continuous gentle subsidence occurring between the Permian transgression and the Early Miocene, a constant heatflow was used for modelling. Burial profiles and maturity indices, such as vitrinite reflectance and Rock-Eval parameters, demonstrate that the Kazhdumi and Pabdeh source rocks reached the onset of oil expulsion during deposition of the Agha Jari Formation between 8 and 3Ma, depending upon the location.
This chronology means that oil migrated from source rocks into pre-existing Zagros structures. Therefore, oil migrated over short distances to nearby traps within well-defined drainage areas, the geometry of which can be deduced from seismic data. Moreover, the Zagros folding induced prominent fracturing which can be observed both at outcrops and in wells. This fracturing, which affects limestones as well as marls, enhanced subvertical migration of hydrocarbons towards the reservoirs.
As a result of this short distance migration, oils can directly be linked to the source rocks which generated them, as shown by oil-to-oil and oil-to-source rock correlations based on stable isotope (δ13C, δ 34S) and biomarker data. Modelling of each drainage area provides estimates of the amount of oil expelled by each source rock. Calculated estimates can then be compared to the actual oil-in-place of the corresponding field. An example of this modelling procedure is given in this paper.
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