Lithic fragments

Lithic fragments Xenolithic—clasts of different chemical class to host Cognate—impact melt fragments... 

Heavily altered, matrix-rich chondritic clasts are very different from the most common lithic clasts in chondrites, which are fragments of chondrules and CAIs or lithic fragments resembling the host rock. They are also unlike rare foreign chondritic clasts that are shocked and appear to have impacted the parent body at high speed after accretion, e.g., an LL chondrite clast in an H chondrite (Lipschutz et al, 1989). 

Figure 11 Ternary diagrams of framework quartz (Q)-feldspar (F)-unstable lithic fragments (L) for sands. Provenance fields from Dickinson et al. (1983) (after McLennan et al., 1990).

Carbonate cements either indirectly enhance or deteriorate the reservoir properties of sandstones. Enhancement of reservoir properties occurs when (i) appreciable volumes of carbonate cements are dissolved, causing the formation of secondary porosity and (ii) small amounts of carbonate cement are evenly distributed in the sandstones to support the overburden weight and prevent the collapse of framework grains and consequent elimination of primary porosity. Souza et al. (1995) demonstrated that a few per cent of dolomite cement is sufficient to prevent the collapse of Aptian reservoir sandstones from Brazil despite the high content of ductile lithic fragments. 

Volcanic rock fragments of intermediate composition are generally the most abundant lithic fragments, averaging 70-90% of all rock fragments (Fig. 5). Chert is the most common sedimentary rock fragment, although some units contain abun-... 

S1-S4 and B1-B3 petrofacies. (A) First-order classification of the sand framework component NCE, non-carbonate extrabasinal CE, carbonate extrabasinal NCI, non-carbonate intrabasinal Cl, carbonate intrabasinal. (B) Classification of the terrigenous sand framework Q, quartz F, feldspars L + CE, fine-grained lithic fragments plus extrabasinal carbonate grains. (C) Classification of fine-grained lithic fragments Lm, metamorphic Lv, volcanic ... 

Lithic fragments are dominantly of metamorphic and plutonic origin. The majority of samples are classified as arkoses (Folk et al. 1970) with an average QFR ratio of 68 22 10. The detrital feldspar grains consist of approximately equal amounts of K-feldspar and plagioclase. The sandstones are extremely clean and lack any recognizable detrital clay matrix. 

Sand and sandstones lose their primary porosity under the influence of three main processes mechanical compaction, pressure solution and cementation. In nature the porosity of graded sands varies on average between 35 and 45%. During the initial stages of burial mechanical compaction of quartz sands may reduce the primary porosity by more than one third and in the case of sands rich in lithic fragments by even more than that. Such a primary reduction of porosity is accompanied by a reorientation and rearrangement of its components and by a plastic deformation of lammellar grains. 

Arenite/wacke Unlike many igneous rocks it is difficult to find a simple relationship between the mineralogy of sandstones and their chemical composition. For this reason the geochemical classification of sandstones does not mimic the conventional mineralogical classification of sandstones based upon quartz-feldspar-lithic fragments. Rather it differentiates between mature and immature sediments. 

Figure 7. Photomicrograph of thin section of the Cold Creek Unit in cross-polarized light The upper portion is dominated by voids (in black) and popcom-shaped growth of secondary calcium carbonate. The lower portion is dominated by lithic fragments of different mineral compositions and few...

Mineralization within the more porous and permeable units, such as lapilli tuff porosity was provided by pumice and lithic fragments. 

Hence, the Edvrards (GC) rock consists mainly of macropores. In contrast, the Fort Riley carbonate is characterized by about an eight times higher fraction of micro- and mesopores than the Edwards (GC). Berea sandstone C3 exhibits the largest fraction of micro- and mesopores (over S %), which is about twice as much as for the Berea sandstone PH2. In Berea sandstone, chert, lithic fragments, partially dissolved feldspars, sheet pores at crystal boundaries and clays contribute to microporosity. 

Not only is the shallowest formation (Fox Hills) plagioclase-rich, but it also has the most porosity attributable to plagioclase dissolution. The Codell Sandstone is highly argillaceous with low permeability, which has retarded the dissolution of framework grains. In all sandstones, secondary porosity attributable to dissolution of K-feldspar is limited to trace amounts. The third form of secondary porosity, which is dissolution of calcite cement and replacement, is sparse (maximum of 1.5vol%) and absent in most samples. Trace amounts of secondary porosity also formed by the dissolution of lithic fragments. 

In the MWX samples, pore quality is closely related to the amount of material containing abundant microporosity, especially dissolved lithic fragments, partially altered feldspars, and clays. Pore quality also correlates fairly well with surface area from BET measurements (Fig. 7a). For all samples examined, pore quality correlated rather well with porosity and with permeability (Figs. 7b and c). It is interesting to note that permeabilities actually correlated better with than they do with... 

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