DIAGENESIS

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INTRODUCTION

Both texture and composition are affected by changes that takes place in sediments after deposition. Wherever these changes occur at relatively low temperature , they are termed as diagenesis.

Von Guembel (1868) was the first to visualize a set of transformation independent of metamorphism , which may change freshly deposited sediments into hard rocks. It was later termed as diagenesis. Such transformation comprise all physico – chemical , biochemical and physical process modifying sediments between deposition and lithification at low temperature and pressure , characteristics of surface and near surface environments (Chilingar et al., 1967).

DIAGENESIS CATEGORIES

In the sandstone, diagenesis is controlled by many factors and processes such as texture, detrital composition and environment of deposition and associated lithologies. Tectonic setting of basin, geothermal gradient, rate and extent deposition and basin subsidence play a significant role. In sandstone, the process can be classified into two broad categories.

Physical and chemical Diagenesis :

Physical diagenesis of the freshly deposited sand results in the compaction of sediments and pore volume reduction due to pressure. At the sediment water interface and at shallow level of burial, physical compaction takes place by the processes of grain rearrangement by rotation , slippage , ductile deformation and grain fracturing without dissolution at grain contacts (Houseknecht , 1987).Continued physical compaction results in increase in number per grain , which passes into a regime of chemical compaction characterized by intergranular pressure solution after considerable depth of burial. The increased geothermal gradient and pressure results in dissolution of grain contacts and their nature from point to long and interpenetrative contacts.

Chemical diagenesis includes relation leading to chemical dissolution, corrosion and cementation. These reactions may start just after the deposition of sand and are controlled by oxidation – reduction at sediment – water or sediment – atmosphere interface. Chemical potential of sediments and pore water chemistry plays a important role in the removal of various unstable phases and precipitation of new stable phases in diagenetic regime. The process of cementation results in loss of porosity as it includes the pore spaces but is reversible in contrast to loss by compaction which is irreversible. The cementing material may be carbonate , silica , iron oxide or clay minerals .The cementation process leads towards the precipitation of new minerals on the grains and into the voids from the pore fluids (saturated with silica , iron oxide or clay minerals).

COMPACTION

The reduction in the bulk volume of rock is called compaction , which occurs in response to four classes of processes viz., grain rearrangement , plastic deformation , dissolution and brittle deformation (Wilson and Stanton , 1994).As sediments are burried compaction takes place. The solid particle of sediments is pressed closer together by the weight of underlying material , and bulk volume is reduce .As a result , the fluid filling is squeezed out and generally migrate slowly through the deposits. The process of compaction is most noticeable in fine aqueous mud, either detrital or chemical , but it occurs to some degree in all sediments.

Physical processe that effect the nature of sand after the deposition leads to a loss of porosity and increase in bulk density primarily by an adjustment to the force of gravitation. The major way in which this happens is compaction through a change in packing by rotation, translation , fracturing and plastic deformation of grains , which results in loss in porosity. Physical rearrangement is greater for well rounded than for angular grains (Fuchtbauer , 1967).The grain to grain and overgrowth to overgrowth contacts were analyzed to work out pre and syncementation compaction on one hand and point cementation point cementation and post cementation compaction on the other. It seems that the compaction started before any cementation events resulting into high point and long grain to grain contacts and continued during silica cementation. The post silica cementation compaction is evidenced by interpenetrative contacts having concavo – convex contact. This observation suggests that the compaction was progressive, which started at sediment – water interface and continued till deep burial.

GRAIN CONTACTS :

Grain to grain contacts of the sediments give an idea about pore spaces reduction and compaction history of the sediments. We have employed two parameters to understand aggregate packing i.e. contact type (Taylor, 1950) and contact index (Pettijohn et al , 1987).

Taylor identified four types of grain contacts in the plane of thin sections – Tangential or Point (P) contacts , Long contact (L) as line , concavo – convex contact © as curve line and Suture contact (S) as serrated interfringing contact. In loosely packed sandstone some grains may not make any contact with other grains , such grains are referred to as floating contact (F).Compaction changes the nature of grain contact and an increased number of long and interpenetrative contacts (C , S) appear at the expense of floating grains and point contacts (F , P).

FRAME WORK CONSTITUENTS :

Taylor (1950) discussed the time relationship between compaction and cementation and suggested that, if cementation has occurs dearly i.e, at the time of deposition, the types of contacts between the original grain and percentage of each type of contact should be the same as in the deposited sand. The framework constituent of the Pachmarhi sandstone of the Singanama area under investigation mainly exhibited by long contacts followed by point – contacts , floating contacts , concavo – convex and suture contacts .The number of contacts around a single grain is counted at grid points , which are classified averaged and classified as contact index (C.I).It is found that, studied sandstone of Pachmarhi sandstone have abundant floating contact grains (21.42 – 69.23 % ) and averaging about (45.5%) followed by concavo – convex contacts (7.69%) , sutured contacts (6.66-9.57%), point contacts (6.6-28.57%) and floating contacts (21-69.23%) (table 7). Dominance of long contact indicates that sand grains do not suffer much pressure solution.