Detailed sedimentological description of conventional cores combined with wireline log response evaluation reveals that both the Ordovician and Devonian successions were deposited in various sub-environments of a wave- and storm-dominated shallow marine clastic shelf, with glacial influence towards the top of the Ordovician. The potential reservoir sandstones consist of upper shoreface to foreshore facies representing third- or higher-order lowstand systems tracts. These LST sandstones are overlain and sealed against offshore mudstone of the ensuing transgressive systems tract, and thus can form stratigraphic traps. Overall, the sandstone units, which occur immediately above and below the thick Silurian graptolitic shales have the best reservoir potential.

Both the Ordovician and Devonian sandstones represent highly mature detritus and consist of a quartzose sand with a typical continental block provenance. In general, the Ordovician sandstones have very poor primary reservoir quality, but are often fractured. Textural evidence shows that compaction is the dominant process of porosity reduction during burial (60 to 85% of porosity loss) in relation to quartz cementation. This is mainly related to the amount of intergranular pressure solution, which in turn is influenced by high heat flow. Fluid-inclusions data suggest two main episodes of quartz cementation: (1) 140 to 175⁰C, interpreted as a Pre-Hercynian burial and compaction event, during which increased burial (4000 – 5200 m) led to the first generation of hydrocarbons (i.e. mainly oil); (2) 175 to <240⁰C, interpreted as reflecting an early Jurassic major thermal event that has consequently engendered thermal cracking of existing liquid hydrocarbons to dry gas, and activated destruction of reservoir properties.

In the Devonian samples, there is only 6.7% (average) of original porosity remaining (XPOR) in thin sections. On average, 49, 24 and 15% of the intergranular volume are respectively occluded by clay, quartz, and carbonate cements. Overall, the Devonian sandstones (Gedinnian, Siegenian, Emsian and Strunian) have undergone highly heterogeneous diagenetic modifications related to the development of grain-coating chlorite, which inhibits the nucleation of quartz cement. In those samples where the clay fraction is dominantly illite, it has promoted intergranular pressure solution and had a dramatic effect on permeability. Owing to latter precipitation of kaolinite, illite, and siderite, an insignificant amount of secondary porosity was gained from dissolution of feldspar in the Gedinnian and Emsian sandstones, in particular.