Changes

Subsurface diagenetic process which produces or increases the porosity is dissolution and secondary porosity occurence. Rock dissolution in water saturated condition produces mold, vein, hole, and channel with or without the breakdown of this appearance (associated with cave and karst generally) and the solution which increases the interparticle porosity. This porosity type divides the origin generally and distinguishes the level based on the type. Generally, the origin shows the similar geological setting, where the porosity builds up and helps geologist to eliminate the unsuitable setting with dissolution. Geological setting where dissolution normally occurs is in the phreatic-meteoric zone, mixing zone, and vadose zone. Dissolution also may occur in the subsurface zone with rocks and water at the outside of chemical equilibrium.

Subsurface diagenetic process which produces or increases the porosity is dissolution and secondary porosity occurence. Rock dissolution in water saturated condition produces mold, vein, hole, and channel with or without the breakdown of this appearance (associated with cave and karst generally) and the solution which increases the interparticle porosity. This porosity type divides the origin generally and distinguishes the level based on the type. Generally, the origin shows the similar geological setting, where the porosity builds up and helps geologist to eliminate the unsuitable setting with dissolution. Geological setting where dissolution normally occurs is in the phreatic-meteoric zone, mixing zone, and vadose zone. Dissolution also may occur in the subsurface zone with rocks and water at the outside of chemical equilibrium.

The interaction between water and rocks where saturated fluide occurs, is causing a stable reaction which changing metastable carbonate or the other stable reaction (recrystallization, includes neomorphism). Weathering and soil formation process in the unconformity involves a combination of diagenetic process that is dissolution, precipitation, biological activity, and neomorphism. Soil and low weathering zone are not always important as the reservoir rocks, because the porosity size of matrix in carbonate is relatively small and have high capillary pressure.

The interaction between water and rocks where saturated fluide occurs, is causing a stable reaction which changing metastable carbonate or the other stable reaction (recrystallization, includes neomorphism). Weathering and soil formation process in the unconformity involves a combination of diagenetic process that is dissolution, precipitation, biological activity, and neomorphism. Soil and low weathering zone are not always important as the reservoir rocks, because the porosity size of matrix in carbonate is relatively small and have high [[capillary pressure]].

General dissolution plays a role as corrosion and increasing porosity in lower burial setting. Lower burial dissolution is called mesogenesis dissolution, following Choquette and Pray<ref name=CP1970> Choquette, P. W. and L. C. Pray, 1970, [http://archives.datapages.com/data/bulletns/1968-70/data/pg/0054/0002/0200/0207.htm Geologic nomenclature and classification of porosity in sedimentary carbonates]: AAPG Bulletin, vol. 54, pp. 200-207</ref> terminology, carbonate porosity classification (Mazullo and Harris, 1992). Saturation depends on CaCO3 in the fluide of burial because the fluide can be rich of CO2, H2S or organically acid. Burial dissolution makes the lower burial-carbonate reservoir to have porosity and permeability which can produce hydrocarbon, though standard “dogma” said that burial carbonate has 5% or low porosity. Increasing of porosity by the diagenetic dissolution produces size of pore with shape with that’s interconnected level widely.

General dissolution plays a role as corrosion and increasing porosity in lower burial setting. Lower burial dissolution is called mesogenesis dissolution, following Choquette and Pray<ref name=CP1970> Choquette, P. W. and L. C. Pray, 1970, [http://archives.datapages.com/data/bulletns/1968-70/data/pg/0054/0002/0200/0207.htm Geologic nomenclature and classification of porosity in sedimentary carbonates]: AAPG Bulletin, vol. 54, pp. 200-207</ref> terminology, carbonate porosity classification (Mazullo and Harris, 1992). Saturation depends on CaCO3 in the fluide of burial because the fluide can be rich of CO2, H2S or organically acid. Burial dissolution makes the lower burial-carbonate reservoir to have porosity and permeability which can produce hydrocarbon, though standard “dogma” said that burial carbonate has 5% or low porosity. Increasing of porosity by the diagenetic dissolution produces size of pore with shape with that’s interconnected level widely.

Compaction and cementation are important in decreasing porosity, because it can be known by calculating the number and type of the contact between grains in sample from different burial depth, so it can be used to estimate how far the compaction can decrease the original intergranular porosity. Rock with even contact within its grains and has some grain contacts in every area and lower porosity than uncompact rock where contacts are usually tangential and rare. If compaction plays more influence than decreasing porosity, there will be more contacts between grain with depth and contact will increase from tangential contact in shallow depth to stylolite contact in deep depth.

Compaction and cementation are important in decreasing porosity, because it can be known by calculating the number and type of the contact between grains in sample from different burial depth, so it can be used to estimate how far the compaction can decrease the original intergranular porosity. Rock with even contact within its grains and has some grain contacts in every area and lower porosity than uncompact rock where contacts are usually tangential and rare. If compaction plays more influence than decreasing porosity, there will be more contacts between grain with depth and contact will increase from tangential contact in shallow depth to stylolite contact in deep depth.

Because the compaction continues with pressure combination and dissolution, so stylolite will be formed. Generally, stylolite will be found more in mud-supported rocks (Dickinson and Saller, 1995) than in grainstone and packstone that generally decreases porosity and permeability (Nelson, 1981). Post-stylolite diagenesis may result porosity and permeability in light rocks (Dawson, 1988). Seismic and wireline log data can’t distinguish cementation, compaction, recrystallization, dissolution and replacement.

Because the compaction continues with pressure combination and dissolution, so stylolite will be formed. Generally, stylolite will be found more in mud-supported rocks (Dickinson and Saller, 1995) than in grainstone and packstone that generally decreases porosity and permeability (Nelson, 1981). Post-stylolite [[diagenesis]] may result porosity and permeability in light rocks (Dawson, 1988). Seismic and wireline log data can’t distinguish cementation, compaction, recrystallization, dissolution and replacement.

====Decreasing with cementation====

====Decreasing with cementation====

Folk (1974) is one of scientists that explains about the importance of Magnesium (Mg), water salinity, and diagenetic environment (vadose, phreatic, or subsurface) as mineralogical control and crystal that form cement. The chronology and micro stratigraphy of cement can be known from the morphology of thin layers of cement. “Cement stratigraphy” shows what happens during burial diagenesis ([[:File:UGM_Subsurface_Fig_1.png|Figure 1]]).

Folk (1974) is one of scientists that explains about the importance of Magnesium (Mg), water salinity, and diagenetic environment (vadose, phreatic, or subsurface) as mineralogical control and crystal that form cement. The chronology and micro stratigraphy of cement can be known from the morphology of thin layers of cement. “Cement stratigraphy” shows what happens during burial diagenesis ([[:File:UGM_Subsurface_Fig_1.png|Figure 1]]).

Because the result of burial and changed composition of water, mineralogy of cement and crystal are also changed. Changed composition of water during burial because of the migration to the mixture of water with in situ interstitial water which will go through various rocks-water reaction. Calcite, dolomite, and other minerals that form cement may be formed, depend on the composition of water and the equilibrium of every minerals. If calcite is formed, usually the shape is big.

Because the result of burial and changed composition of water, mineralogy of cement and crystal are also changed. Changed composition of water during burial because of the migration to the mixture of water with in situ interstitial water which will go through various rocks-water reaction. Calcite, [[dolomite]], and other minerals that form cement may be formed, depend on the composition of water and the equilibrium of every minerals. If calcite is formed, usually the shape is big.

Decreasing porosity from sementation can be identified by the cross-cutting relationship in “cement stratigraphy”. Isopachous cement with rim may be formed on early sementation in marine phreatic zone. Rougher cement may decrease more porosity. Residual effective porosity may be connected to poikilotopic cement that’s formed in deep burial environment. All cements may be crossed by joint that’s filled with mineral or burial-exotic deep cement. Exotic mineral cement and mineralized joints become the indicator of new permeability, followed by migration of mineralization fluid. Exotic fluid is generally associated with hydrocarbon migration and chances of forming new permeability is always as joint that’s formed after diagenesis process.

Decreasing porosity from sementation can be identified by the cross-cutting relationship in “cement stratigraphy”. Isopachous cement with rim may be formed on early sementation in marine phreatic zone. Rougher cement may decrease more porosity. Residual effective porosity may be connected to poikilotopic cement that’s formed in deep burial environment. All cements may be crossed by joint that’s filled with mineral or burial-exotic deep cement. Exotic mineral cement and mineralized joints become the indicator of new permeability, followed by migration of mineralization fluid. Exotic fluid is generally associated with [[hydrocarbon migration]] and chances of forming new permeability is always as joint that’s formed after diagenesis process.

Scholle and Halley (1985) said that generally there’s just a little loss of porosity in zones near the surface water circulation (vadose, meteoric-phreatic, mixing zone). Transition of carbonate sediments are very porous and good cemented. Rocks with low porosity are dominated by subsurface processes. Scholle and Halley (1985) also explains about the important point of burial diagenesis. In reality, condition of fabric and texture sedimentation is combined together with early diagenetic processes like dissolution, cementation, and dolomitization that become the main factor which controls the distribution of subsurface porosity and permeability.

Scholle and Halley (1985) said that generally there’s just a little loss of porosity in zones near the surface water circulation (vadose, meteoric-phreatic, mixing zone). Transition of carbonate sediments are very porous and good cemented. Rocks with low porosity are dominated by subsurface processes. Scholle and Halley (1985) also explains about the important point of burial diagenesis. In reality, condition of fabric and texture sedimentation is combined together with early diagenetic processes like dissolution, cementation, and dolomitization that become the main factor which controls the distribution of subsurface porosity and permeability.

# Many carbonate rocks porosity was formed at shallow-depth burial

# 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)

# 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

# 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. [[:File:UGM_Subsurface_Fig_3.png|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.

In addition to relation of the porosity and depth forming, there are relationship between porosity break-up and depth. [[:File:UGM_Subsurface_Fig_3.png|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.

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.

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.

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.

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.

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.

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.

(A) The accumulation of hydrocarbons in the stratigraphic trap as a result of loss updip porosity. Calcite cementation happened to overburden contact oil / water. (B) With folding contact of oil / water layer is at end position a reservoir that has no relation with the structure<ref>Aquitaine, E., 1982, Exploration for Carbonate Petroleum Reservoirs: New York, John Wiley & Sons, Inc.</ref>]]

(A) The accumulation of hydrocarbons in the stratigraphic trap as a result of loss updip porosity. Calcite cementation happened to overburden contact oil / water. (B) With folding contact of oil / water layer is at end position a reservoir that has no relation with the structure<ref>Aquitaine, E., 1982, Exploration for Carbonate Petroleum Reservoirs: New York, John Wiley & Sons, Inc.</ref>]]

[[:File:UGM_Subsurface_Fig_4.png|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.

[[:File:UGM_Subsurface_Fig_4.png|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]].

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).

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).