Impact of contrasting paleoclimate on carbonate reservoir architecture: Cases from arid Permo-Triassic and humid Cretaceous platforms in the south and southwestern Iran

Hamzeh Mehrabi, Hossain Rahimpour-Bonabn, Amir Hossain Enayati-Bidgoli,
Behrooz Esrafili-Dizaji
School of Geology, College of Science, University of Tehran, Iran
Abstract

Carbonate platforms formed and evolved in different climatic conditions (i.e. arid vs. humid) show diverse features which are reflected in their internal reservoir architectures. The Permo-Triassic Dalan– Kangan and Upper Cretaceous Sarvak carbonates host huge hydrocarbon accumulations in the south and southwest Iran. These successions, along with their equivalents in the Middle East, are among the best examples of carbonate platforms formed and evolved in two different climatic conditions (i.e. arid and humid, respectively).

Disparate climatic conditions had led to drastic changes in their facies characteristics, later diagenetic alterations, reservoir characteristics, and architecture. To investigate the impact of paleoclimatic conditions on their reservoir characteristics, integrated sedimentological, geochemical and reservoir zonation studies are carried out on (core) samples from selected successions in seven oil and gas fields across the Dezful Embayment, in SW Iran, and in the Persian Gulf. Integration of detailed sedimentological studies with petrophysical evaluations has resulted in the perception of some fieldscale variations in these carbonate sequences.

 Introduction

In carbonate successions, reservoir characteristics (i.e. porosity and permeability) are controlled substantially by depositional patterns and later diagenetic alterations (Schlager, 2005; Ahr, 2008).

The main depositional parameters controlling reservoir properties and architecture of carbonates are the depositional fabrics, the genetic platform configuration (geometry), sea-water fluctuations and composition as well as paleoclimatic conditions (Sun and Esteban, 1994; Petty, 2005; Ehrenberg et al., 2007).

In addition, diagenetic alterations (especially eo-genetic or near surface diagenetic processes) depend directly or indirectly on primary physiochemical nature of the carbonate sediments (i.e. their primary mineralogy) are controlled by their depositional settings (such as tropical vs. temperate environments) and fabrics (i.e. grain vs. mud dominated).

Thus, in these carbonate sediments, pathways of diagenetic evolutions and so history are primarily directed by depositional characteristics (Moore, 2001; Braithwaite et al., 2004; Ahr, 2008). The Upper Cretaceous and Permo-Triassic carbonate reservoirs of south and southwestern Iran (Fig.1) are chosen for this study as two examples of humid and arid carbonate platforms, respectively, in order to investigate the paleoclimatic impact on their reservoir quality evolutions and internal geometries.

Because of the shallow depth of the carbonate factory (less than 15m water depth in which carbonate production is at maximum level), these sediments are subjected to the complex history of the eustatic, and also even local, sea-level fluctuations and tectonic activities (Moore, 2001). Along with climatic and physiochemical controls of the depositional setting, timing and duration of subaerial exposure could exert strong influences on the diagenetic processes that occurred along unconformities (Mazzullo and Chilingarian,1992; Rahimpour-Bonab et al., 2012a, 2012b ).

The Permo-Triassic Dalan–Kangan and Upper Cretaceous Sarvak carbonates (Table 1) are two important hydrocarbon reservoirs in the Middle East (Fig. 2), hosting large volumes of gas and oil in both onshore and offshore fields in south and southwestern Iran, including the Dezful Embayment in SW Iran and the Persian Gulf (e.g. Motiei, 1993; Sharland et al., 2001).

Primarily, the reservoir quality of these carbonates appears to have been controlled by the prevailing paleoclimate to some extent, which exerted a major control on both the depositional patterns and diagenetic pathways (Esrafili-Dizaji and Rahimpour-Bonab, 2009; Mehrabi and Rahimpour-Bonab, 2014).

These two carbonate reservoirs are well-known examples of carbonate systems, formed under arid and humid tropical to subtropical climates, respectively (Fig. 1A). However, the roles of contrasting palaeoclimatic conditions on reservoir architecture of these two important reservoirs are not still understood.
In this paper, our main objectives are to document and explain the facies changes and diagenetic alterations in the context of climatic/environmental evolutions in two selected Permo-Triassic and Upper Cretaceous carbonate reservoirs in several subsurface sections in the south and southwestern Iran. So, our attempt is to present the impacts of contrasting tropical vs. arid paleoclimatic conditions on the distribution of reservoir and non-reservoir zones in these carbonate sequences. In brief; how distinct reservoir architectures formed by contrasting paleoclimatic conditions?
Table 1
Important characteristics of the studied intervals, including the Permo-Triassic Dalan–Kangan and Upper Cretaceous Sarvak reservoirs in the south and southwest of Iran.

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