Structural-mineralogical alterations of sandstones in rock-water and rock-water-oil systems within oil-water contact

. Structural-mineralogical alterations of sandstones under conditions of natural water-oil interaction within Jurassic sedimentary sequences in south-eastern Nurolsk sedimentary basin (Tomsk Oblast, Western Siberian, petroleum province) depend on their predominance in the fluid system and the dissolution-resedimentation process itself. Oil inflow and associated acid solutions into the reservoir resulted in rock-water disequilibrium: unstable minerals decompose and new stable mineral phases form. Under conditions of oil-filling reservoir, terrigenous reservoir alteration reveals zonal behavior. In the oil saturation zone (rock-water-oil system) solution and secondary mineralization are minimized; in the transition zone of water-oil contact (rock-water-oil and rock-water systems) dissolution is well-developed and accompanied by intensive kaolinitization; in edge water zone (rock-water system) no dissolution and predominating quartz-carbonate cementation.


Introduction
Rock-water-oil system constitutes an integral part of the widely-distributed rock-water system on the Earth. On the one hand, this system is related to the sweeping processes between water and rock-sediments buried during sedimentogenesis and altered during catagenesis; on the other hand, the sweeping process in the reservoir during the migration of hydrocarbons of the oil sequence and the formation, generation and disintegration of the reservoirs. The article discusses the structural-mineralogical alterations of sandstones in layer-arch reservoir traps, oil-filled not to spill, generating oil-water contact (OWC), involving oil-water transition zones of different thicknesses.
Research target includes Tumen formation (J2tm) sand sediments, exposed by drilling Gerasimovskoe oil-gas-condensate field (south-eastern Nurolsk sedimentary basin in Western Siberian oil-gas bearing province, Tomsk Oblast), where bottom water forming OWC underlies the oil reservoir. Stadial rock alteration level is related to catagenesis of МК2-3 gradation.
Allothigenic (quartz, feldspar) and authigenic (carbonates, kaolinite, pyrite) minerals were examined in different thicknesses of the sand reservoirs, which reflected the most contrasting rock alteration processes under formation conditions of the oil reservoir within rock-water and rock-water-oil systems.

Results and discussion
Reservoir formation in Jurassic sedimentary sequences is associated with successive pulsed oil inflow into the reservoir trap, conditioned by superimposed-epigenetic structuralmineralogical zonation resulting from rock-water-inflow oil interaction [1].
Feldspar is substituted by pelite and sericite (more than 50%). Based on X-ray fluorescence analysis including luminescence range of 400-470 mn, reflecting the crystalline defects in SiO4 3and AlO4 4-, the luminescence range of maximal 570 mn and 700 mn was observed in the X-ray fluorescence spectrum, being characteristic of impurity cations Mn 2+ and Fe 3+ , conditioned by hydromication.
According to the characteristic features of fluid saturation and structural-mineralogical rock alteration, three zones were identified in the formation cross-section, successively downward: 1 -oil saturation zone and capillary bound water (rock-water-oil system); 2 -OWC transition zone including mobile oil and water: 2A -upper subzone with mobile oil and water (rock-water-oil system); 2B -bottom subzone with mobile water (rock-water system); 3 -discharge zone of water solutions (rock-water system).
Pore water, saturated by unaffected by the oil reservoir rocks exhibits poor alkaline character and is in equilibrium with carbonate and other mineral rocks [2].
Sandstones, from oil saturation zone and capillary bound water (rock-water-oil system) zones, are sensitive to oil intrusion and its associated water fluids into pore spaces.
Quartz is poorly dissolved on the periphery of the zone and sometimes locally degenerated.
Feldspar is dissolved on the periphery and inside the grain (along cleavage crack), being replaced by pelite (kaolinite) and sericite (hydromica) to various extent (up to 25%). Dissolution and hydromicatization are reflected on the X-ray fluorescence spectrum as increased concentration and high (25 units) structure defect intensity (SiO4 3and AlO4 4-), corresponding to band O 2within 400-470 mn range. Hydromicatization is recorded as intensive (up to 50 units) luminescence of impurity defects Fe 3+ (maximal luminescence 710 mn) and Mn 2+ (maximal -570 mn).
Under conditions of oil-filling trap, rock-water system transforms into rock-water-oil system. Under the action of acid water, accompanied by migrating oil and containing organic acids and different gases (CO2 and H2S) [3,4], alkaline environmentally resistant carbonates, feldspar and partially quartz dissolve. Chemically pure calcite dissolves completely, while isomorphously substituted siderites are deprived of Са 2+ and Mn 2+predominately mobile cations.
Both sandstones from the transient WOC zone and mobile oil and water in the rock-oilwater system embrace intensive dissolution and secondary mineralization. Subzone 2A (rock-water-oil system) is characteristic of pyrite-kaolinite and 2B (rock-water system)kaolinite secondary mineralization.
Quartz is dissolved and reveals thinly-waved surface, no regeneration. Feldspar are intensively dissolved (50-55 %) and substituted by kaolinite and sericite. As a result of secondary mineralization, the X-ray fluorescence spectrum reflected intensive luminescence of structural defects (center -type O 2-) and increasing intensive luminescence of impurity defects Fe 3+ (710 mn) and Mn 2+ (570 mn).
Pore space is the result of rejuvenated dissolution of residual sedimentagenous and diagenetic intergranular and intragranular pores, as well as newly-formed micro-pores on the surface and within fragments and micro-pores in kaolinite cement.
In the upper subzone (2A) WOC (rock-water-oil system) oil and water are continuously in proximate contact and interaction. Anaerobic oxidized sulfur-reducing oil bacteria transforms into bitumen, in which, at the expense of the consolidation of water-dissolved Fe 2+ and liberated sulfur from sulphate, pyrite is formed (FeS2). Formed acid solutions intensively dissolve fragments and cement, resulting in the fact that calcite and dolomite fade out of the system, while siderite is free of soluble impurity cations. Kaolinitization (Al2Si2O5(OH) intensifies due to kaolinite synthesis from enriched Al 3+ solutions [4]. In the bottom subzone (2B) WOC (rock-water system), where circulating acid waters enter into subzone 2A, pyrite does not form. However, intensive rock dissolution and formation of monomineral kaolinite cement continues.
Cementation zone is formed in sandstones located in water solution discharge zones (rock-water system), where increased amount of secondary quartz and carbonates are identified.
Quartz is formed on clastic grains with regenerated hems and sometimes even boundaries of 0.006 to 0.02 mm thickness.
Carbonates corrode quartz, substitute feldspar and compose cement. X-ray fluorescence spectrum of feldspar-substituted calcite exhibited weak structural defects of SiO4 3and AlO4 4and intensive luminescence of maximal 620mn, which is characteristic of Mn 2+ , found in carbonates.
With distance from WOC, water-oxidation products cease to interact with water, acid intensity of solution occurs both in neutral conditions and alkaline environment, while in the hydrodynamic closed rock-water system successive secondary quartz deposition [5,6] and isomorphically substituted carbonates occur.
Research results illustrated in Table 1  Table 1. Structural-mineralogical alterations of sandstones in rock-water and rock-water-oil systems within generating oil-water contact zones.

Zone / Subzone System Transformations
Oil saturation and capillary bound water Rock-wateroil Rock: limited dissolution processes, substitution (feldspar) and mineralization (quartz, kaolinite), predominate intragranular pores. Water: blending alkaline (porous) water and acid water (migrating and resulting from oil oxidation). Oil: limited oxidation on pore (interstitial) water contact.