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Subsurface diagenesis and its relation with diagenetic trap on carbonate rocks reservoir (view source)
Revision as of 16:42, 13 July 2015
, 16:42, 13 July 2015no edit summary
==Relation between porosity changes and depth==
==Relation between porosity changes and depth==
The question about in what depth that porosity of carbonate rocks formed seems like a simple question. But in fact, it’s impossible to calculate the relationship between porosity and depth.
[[FIG 2]]
The important point which showed from Figure 2 is :
# Many carbonate rocks porosity was formed at shallow-depth burial
# A few porosity was formed at more than a hundred meters depth cause the solution produce sthylolite not the pores at homegenic rocks. There are many exception where the burial dissolution in heterogenic rocks produce pores (Mazullo; Loucks and Budd, 1981)
# The cracking porosity can occurs in a certain depth but it’s seldom that more than a percents
In addition to relation of the porosity and depth forming, there are relationship between porosity break-up and depth. Figure 3. showing a main process of porosity break-up depends on the depth where it’s process occurs. These diagram is subjective, but can showing more of porosity breaking up thats occurs near the earth surface where the sediment submiting, thats have compaction and early cementation.
[[FIG 3]]
Process that can breaking up the porosity there are late-cementation which generally related with pressure solution at the same rocks or in adjacent and pressure solution which reducing the porosity by solution in grain contact or pore type changing along sthylolite. The important point from there are many porosity showed reduction at shallow depth by the easily predictable process. Porosity that forming at the shallow depth can moving to the deep subsurface may be preserve without sthylolitization. If there are occurs sthylolitization, the last cementation occurs generally. (Wong and Oldershaw, 1981)
==Identification subsurface diagenesis that form diagenetic trap in carbonate rocks reservoir==
Stratigraphic trap divided into two primary stratigraphic trap and secondary stratigraphic trap (Levorsen, 1954). Primary stratigraphic trap formed during deposition or diagenesis rock include formation of lens, facies changes and reef. Secondary stratigraphic trap is formed after deposition processes and diagenesis such as leaching and cementation but The main is unconformity.
The primary stratigraphic traps are a direct product of the deposition environment which is characteristic of the composition in the reservoir and the condition after deposition. Konkav upper surface on the trap has a effective pore cavities which is result of primary sedimentation process. This trap also called "depositional" trap and "diagenetic" trap.
There are two groups of primary stratigraphic traps in carbonate rocks which is essential to produce oil and gas. The first group form porous rocks facies either as lithofacies or biofacies surrounding shale, limestone or dolomite, as well as lenses containing residual carbonate organisms called biostrome. The second group in the form of porous or carbonate mound shaped lens consisting of mixture of in situ organisms and rock surrounding the so- called organic reef or bioherm.
One of stratigraphic traps which formed associated with subsurface diagenesis is porous carbonate facies. This trap group could evolve locally and regionally. The most common type is formed from the process dolomitisation where magnesium carbonate limestones deposited with less volume than the calcium carbonate replaced by a solution, resulting in porous and permeable rock.
In addition, the stratigraphic trap that can be influenced by subsurface diagenesis is a reef or bioherm (Landes, 1946). The main reason of oil production from some reef which rest on high porosity and high permeability reservoir rock. This porosity can be formed early or can be induced. Porosity early formed between the presence of abandoned spaces where the organisms life and the empty cavity between the outer wall shells on the set various organisms. While the induced porosity is generated from washing mass coral reefs by the circulation of water when approaching the surface, solution which exceeds precipitation during dolomitisation and can also be from reef fracture because the movement of the earth.
Diagenesis below the surface also has a relationship with the migration. When migration occurs early and precedes episodes of tilting or folding it takes prudence on the possible diagenesis trap (Wilson, H.H., 1975, 1977). Migration earlier indicated by some diagenesis carbonate sequences which suffered beforehand as cementation.
Figure 4 illustrates the sequence of events associated with diagenetic trap formation. In the example shows the mechanism original trapping with reduced porosity updip as a result of changes facies of grainstone oolitic into calcareous mudstone.
[[FIG 4]]
At the contact of oil / water in the reservoir is often precipitate calcite cement extensively. This phenomenon generally occurs in some carbonate reservoir which showed no evidence of cementation except in contact area of oil / water. The calcite cement occurrences associated with the reaction of sulfate reducing bacteria which can be explained in the following reaction (Friedman and Sander, 1978).
Formation water organic material calcite cement:
:<math>\text{Chemical Equation}</math>
Generally calcite cement filled pore cavities at the contact of oil / water, trapping oil (sealing oil) between reservoir and reservoir formation impermeable here in after the movement of fluid. When cementation occurs extensively contact in the oil / water followed by a period of tilting or folding it no loss of oil from the reservoir and there is no transfer of oil to structural highs. When the trap diagenesa formed between rock sequences, exploration drilling on the structural highs will not succeed. To determine the location of potential diagenetic trap required sequence are not tilted or not bent by using information from seismic so that can be found trap earlier position that is covered by cementation at the contact of oil / water.
==References==
* Ahr, W.M., 2008, Geology of Carbonate Reservoirs: United States, A John Wiley & Sons, Inc.
* Anderson, T. F. and Arthur, M. A., 1983, Stable isotopes of oxygen and carbon and their application to sedimentologic and paleoenvironmental problems, in M. A. Arthur (Ed.), Stable Isotopes in Sedimentary Geology: SEPM Short Course No. 10
* Aquitaine, Elf, 1982, Exploration for Carbonate Petroleum Reservoirs: New York, John Wiley & Sons, Inc.
* Back, W., Hanshaw, B. B., Plye, T. E., Plummer, L. N. and Weidie, A. E., 1979, Geochemical significance of groundwater discharge and carbonate solution to the formation of Caleta Xel Ha, Quintana Roo: Mexico, Water Resources Res.
* Bathurst, R. G. C., 1975, Carbonate Sediments and their Diagenesis: New York, Elsevier Science Publ. Co.
* Bishop, Michele G., 2000, South Sumatra Basin Province, Indonesia: The Lahat/Talang Akar- Cenozoic Total Petroleum System: U.S. Department of The Interior, U.S. Geological Survey
* Boggs, Sam Jr., 1992, Petrology of Sedimentary Rocks: New York, USA, University of Oregon, Macmillan Publishing Company
* Boles, J. R. and Franks, G., 1979, Clay diagenesis in Wilcox sandstone of southeast Texas: implications of smectite diagenesis on sandstone cementation: J. Sediment. Petrol. 49
* Burst, J. R., 1969, Diagenesis of Gulf Coast Clayey Sediments and Its Possible Relation to Petroleum Migration: AAPG Bulletin 53
* Carpenter, A. B., 1978, Origin and chemical evolution of brines in sedimentary basins: Oklahoma, Geol. Surv. Circ. 79
* Chapman, R.E., 1983, Petroleum Geology: Amsterdam, Elsevier Science B.V.
* Choquette, P.W., dan James, N.P., 1990, Limestones - The Burial Diagenetic Environment dalam Diagenesis: Canada, The Runge Press Ltd.
* Choquette, P. W. and Pray, L. C., 1970, Geologic nomenclature and classification of porosity in sedimentary carbonates: AAPG Bulletin 54
* Chilingarian, Mazullo, Rieke, 1992, Carbonate Reservoir Characterization: A Geologic- Engineering Analysis, part I: Amsterdam, Elsevier Science B.V.