Siliciclastic rocks

With a few exceptions reservoir rocks are sediments. The two main categories are siliciclastic rocks, usually referred to as elastics or sandstones , and carbonate rocks. Most reservoirs in the Gulf of Mexico and the North Sea are contained in a clastic depositional environment many of the giant fields of the Middle East are contained in carbonate rocks. Before looking at the significance of depositional environments for the production process let us investigate some of the main characteristics of both categories. 

The interpretation of lithogeochemical data from basaltic structural domes is complicated by lithological changes associated with the transition from basalt to overlying siliciclastic rocks, as well as by the polydeformed nature of the host sequence. Ferroan carbonate alteration is well developed, and low-level Au enrichment extends for a considerable distance away from zones of economic interest. Arsenic and Sb/AI anomalies are restricted to within approximately 10 m of mineralized zones. Sericite alteration is indicated by Na depletion and K enrichment in basalt within 20 to 40 m of mineralized zones. A number of other elements, including Mn, P, S, Zn, Mo, Cu, Se and Ba, are variably enriched within the rocks hosting Au mineralization, but it is not clear whether elevated concentrations of these elements are a product of syn-sedimentary exhalative activity or result from later hydrothermal alteration. 

Spalletti, L.A., Merodio, J.C., Matheos, S.D. 2008. Sedimentary petrology and geochemistry of siliciclastic rocks from the upper Jurassic Tordillo Formation (Neuque n Basin, western Argentina) Implications for provenance and tectonic setting. Journal of South American Earth Sciences, 25, 440-463. 

Milliken K. L. (1994b) The widespread occurrence of healed microfractures in siliciclastic rocks evidence from scanned cathodoluminescence imaging. In Rock Mechanics, Models and Measurements, Challenges from Industry, Proceedings of the 1st North American Rock Mechanics Symposium (eds. P. P. Nelson and S. E. Laubach). Balkema, Rotterdam, 1155pp. 

Morad S., Ketzer J. M., and De Ros L. F. (2000) Spatial and temporal distribution of diagenetic alterations in siliciclastic rocks implications for mass transfer in sedimentary basins. Sedimentology 47(Suppl. 1), 95-120. 

Wintsch R. P. and Kvale C. M. (1994) Differential mobility of elements in burial diagenesis of siliciclastic rocks. J. Sedim. Res. 64, 349-361. 

Hutcheon, I. (1983) Aspects of the diagenesis of coarsegrained siliciclastic rocks. Geosci. Can., 10, 4-14. 

Studies involving saddle dolomite are grouped into three categories based on their host lithology carbonate, sandstone and carbonate-sandstone mixtures. Most saddle dolomite has been reported from carbonate rocks (Table 1), particularly units associated with MVT ore deposits, whereas only a few studies provide detailed information on saddle dolomite in siliciclastic rocks. 

In siliciclastic rocks, many physical properties (elastic wave velocity, electrical resistivity, permeability) show a strong correlation to porosity. In carbonate rocks, correlations superimposed by the heterogeneous pore distribution, pore type, pore connectivity, and grain size (Westphal et al., 2005). Table 1.3 compares some prominent properties of the two main groups of reservoir rocks. 

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