Diagenetic Controls on the Early to Late Bathonian Fort Member Sandstone of Jaisalmer Formation, Western Rajasthan

Abstract

Petrographic analysis, Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD) were used to examine the various diagenetic controls on reservoir quality of the Fort Member Sandstone (FMS), Jaisalmer Formation in western Rajasthan. Diagenetic processes include mechanical compaction, cementation, pressure solution and dissolution of framework grains. The main diagenetic cements are carbonate, ferruginous, silica cement and authigenic clays. Cements occur as coatings around the detrital grain boundaries, pore fillings and pore linings. Compaction and cementation are common factors responsible for the reduction of porosity and permeability. The dissolution of detrital feldspar grains was the main drive for porosity enhancement in the FMS. The corroded contacts between successive carbonate phases and quartz grains, followed by the dissolution of carbonate cements were also responsible for the secondary porosity. The main porosity preservation in the FMS is due to scattered patches of carbonate cement, which prevented the compactional collapse of the framework. The relationship between the intergranular volume (IGV) and cement volume indicates a minor role of compaction in destroying the primary porosity. The widespread occurrences of carbonate cement suggest that the FMS lost a significant amount of primary porosity at an eodiagenetic stage. In addition to carbonate, authigenic clays like chlorite and kaolinite occur as pore-filling and pore-lining cements. The pore-filling chlorite resulted in a considerable loss of porosity, while the pore-lining chlorite may have helped in retaining the porosity by preventing the precipitation of syntaxial silica overgrowths. Paragenetic sequences are established to understand the pathways of diagenetic evolution and their impact on reservoir quality of the FMS.Please check and confirm if the authors and their respective affiliations have been correctly identified. Amend if necessary.Confirmed the respective authors affiliations and correctly identified.

Appendix 1

Contact Index (CI) = the average number of contacts per grain irrespective of nature of contacts.

Packing Proximity (Pp) = is expressed as percentage of grain to grain contacts in a traverse:

$${\text{Pp}} = \frac{\text{q}}{\textrm{n}} * 100$$

where q is the number of grain to grain contact and n is total number of grains.

Contact Strength (Cs) = is quantified by following formula:

$${\text{Cs}} = \frac{{{\text{a}} + 2{\text{b}}}}{{{\textrm{a}} + {\text{b}}}}$$

where a, is the number of point contact and b, is the number of all other contacts including long, concavo-convex and sutured contacts.

Consolidation Factor (Cf) = is expressed as:

$${\text{Cf}} = \frac{{{\text{F}} + 2{\text{T}} + 3{\text{L}} + 4{\text{C}} + 5{\text{S}}}}{500} * 100$$

where F, T, L, C and S are the percentages of floating, tangential, long, concavo-convex and sutured contacts respectively

Weight Contact Packing (WCP) = $\frac{{{\text{a}} + 2{\text{b}} + 4{\text{c}} + 8{\text{d}} + 16{\text{e}}}}{{{\textrm{a}} + {\text{b}} + {\text{c}} + {\text{d}} + {\text{e}}}}$

where is floating contact (a), point contact (b), long contact (c), concave convex contact (d) and sutured contact (e).

Intergranular Volume (IGV)  = Sum of remaining primary pore spaces, volume of pore filling cements and depositional matrix (Paxton et al. 2002). This is equivalent to the term “pre-cement primary porosity” and the term minus-cement porosity commonly found in earlier publications.

Compactional Porosity Loss (COPL) = amount of original porosity lost by compactional processes (expressed as a percentage of the original rock volume).

$$({\text{COPL}}) = {\text{Pi}}\left\{ {\frac{{(100 - {\text{Pi}}) * {\text{IGV}}}}{{100 - {\text{IGV}}}}} \right\}$$

where Pi is the initial depositional porosity (=40%) and IGV is intergranular volume (sum of remaining primary pore spaces, volume of pore filling cements and depositional matrix (Paxton et al. 2002).

Cementational Porosity Loss (CEPL)  = amount of original porosity lost by precipitation of intergranular cement (expressed as a percentage of the original rock volume).

$$({\text{CEPL}}) = ({\text{Pi}} - {\text{COPL}}) \times \frac{\text{Tc}}{\textrm{IGV}}$$

where Tc is total cement and IGV is intergranular volume.

$${\text{Icompact}} = \frac{\text{COPL}}{{{\textrm{COPL}} + {\text{CEPL}}}}$$