THE INFLUENCE OF CEMENTATION ON THE RESERVOIR QUALITY OF THE RISHA SANDSTONE MEMBER (UPPER ORDOVICIAN),

RISHA GASFIELD,  NE JORDAN

M.  Y.  Tamar-Agha*

* Department of Geology, College of Science, University of Baghdad, Jadiriya, Baghdad, Iraq.

present address: Department of Geology, College of Science, Salahaddin University, Erbil, Iraq.

email: mtamaragha@yahoo.com

Shallow-marine sandstones of the Upper Ordovician Risha Member of the Dubaydib Formation constitute important reservoir rocks for natural gas in the Risha area of NE Jordan. However, reservoir quality is frequently reduced by diagenetic effects including extensive cementation. This paper reports on the petrology of the Risha Sandstones and the nature of diagenetic alteration, based on standard petrographic analyses of 147 core samples collected from nine wells at Risha gasfield. In addition ten representative samples were investigated using scanning electron microscopy (SEM).

The sandstones comprise subarkoses and quartzarenites which are texturally submature to supermature. Petrographic analyses suggest a continental provenance, most probably from precursor plutonic rocks from the Arabian Shield. A generalized model of paragenesis includes early compaction and development of syntaxial feldspar overgrowths with quartz cementation of variable intensity. Carbonate cements occur as replacement and pore-filling phases. Dolomite and siderite cementation occurred during later-stage burial, together with pressure solution and the growth of authigenic clays. Most of the cements originated from meteoric or compaction-derived waters. Separate pathways of sandstone diagenesis were recognized for the texturally supermature and submature sandstones, both of which were facies-controlled.

Occlusion of most of the porosity preceded hydrocarbon accumulation, which is therefore restricted to areas where cementation was minimized or where cements were dissolved.

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