REVIEW OF THE LOWER-PALAEOZOIC RESERVOIR PROSPECTIVITY OF W. IRAQ WITH REFERENCE TO JORDAN

Adnan Aqrawi

doi:10.46717/igj.53.2b.2rs-2020-08-02

Abstract

The Palaeozoic basin of Jordan and Iraq occupies the NW part of the Arabian Plate. The Lower Palaeozoic succession is the most under-explored succession in Iraq, particularly in western Iraq where it is considered the most promising target for future exploration. The western Iraqi subsurface and Jordan geology are characterized by similar major chrono-stratigraphic rock units. In western Iraq, the Akkas-1 well is the only gas and condensate discovery in sandstone reservoirs of Early Silurian and Late Ordovician age. Lower Silurian sandstones of the Akkas Formation are an oil reservoir characterized by a mean porosity of 6.5% and permeability of 0.2mD. Underlying Upper Ordovician sandstones of the upper part of the Khabour Formation are also reservoir, but for sweet gas and condensate. The latter are comparable to both the Risha and Wadi Sirhan discoveries in Jordan. Several deeper potential sandstone reservoirs (of older Ordovician and Cambrian ages) are also present across the region. Equivalent Ordovician age Jordanian reservoirs are primarily developed in homogeneous, highly mature quartz arenites of storm-dominated clastic-shelf stratigraphies. Within the Risha Field and the Wadi Sirhan area, are also end-Ordovician (Hirnantian) glaciogenic and periglacial sandstones with more variable reservoir quality characteristics compared to the underlying quartz arenites. These often comprise highly immature arkoses (as diamictites) that provide a mineralogically much less stable assemblage in which there is a possibility of developing secondary porosity. Sedimentological and XRD mineralogical studies suggest that diagenesis is an important factor in developing of reservoir quality. Nevertheless, reservoir quality is a major exploration risk where burial was significant that led to quartz overgrowth and clay cementation. It is notable that some sandstone reservoirs have chlorite-clay coatings, which prevented formation of the quartz overgrowth and preserved good reservoir quality. The widespread occurrence of associated source and seal rocks together with the proven and potential reservoirs, suggests that Lower Palaeozoic prospects below the deserts of western Iraq are promising targets for future hydrocarbon exploration.

Highlights

North-western Arabian Plate (Fig. 1A) is considered the most promising exploration region for Lower Palaeozoic reservoirs
The western Iraq and Jordan are characterized by similar subsurface major chrono-stratigraphic rock units (Figs. 2A and 3)
The thickest stratigraphic sequence lies on the border area between Jordan and Iraq, whilst to the south, the same stratigraphy thins up against the Arabian Shield, as it thins westwards towards the Dead Sea Rift

Summary

Introduction

North-western Arabian Plate (Fig. 1A) is considered the most promising exploration region for Lower Palaeozoic reservoirs. Due to limited subsurface penetration and data availability of such succession in western Iraq as well as at the northern Iraq outcrops, a short review of stratigraphic, sedimentological and mineralogical studies of selected outcrop and borehole rock-samples of Jordan was carried out. The latter characteristics of every rock-unit were proposed for the potential correlated equivalent rock-unit in the western Iraq succession within a third-order stratigraphic framework (Fig. 3). The latter 3rd order sequences were previously identified by Aqrawi et al (2010) for each regional Arabian Plate (AP) Megasequence of Sharland et al (2001 and 2004), for AP2 and AP3

Objectives

Methods

Results