Subsurface diagenesis and its relation with diagenetic trap on carbonate rocks reservoir

Wiki Write-Off Entry
Student Chapter	Universitas Gadjah Mada
Competition	June 2015

Relation between subsurface diagenesis with porosity changing

Increase of porosity

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.

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 (1970) 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.

Decreasing porosity

Decreasing with compaction

Mechanical compaction is resulted from overburden pressure during burial or from tectonics. Compaction causes exaggerated appearance when dissolution and compaction happen together and form stylolite when the pressure and dissolution is even. Graphic which shows decreasing pressure with increasing depth (Schmoker and halley, 1982) says that compaction plays bigger influence than cementation in decreasing porosity. Cementation happens during burial, so decreasing porosity and depth don’t have a function in a variable.

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.

Decreasing with cementation

Cementation happens some times in carbonate rocks diagenesis, starts with cementation in marine environment after sedimentation until vadose environment, shallow, medium, and deep burial. Minerals and crystals will form carbonate cements that changes water chemically and depositional environment changes from marine phreatic to meteor phreatic or from shallow to deep below the surface.

FIG 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 (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.

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.

Longman (1980) clearly rejects the importance of burial diagenesis process such as compaction and cementation that decrease the volume of porosity and quality during burial of a sequence. Migration of hydrocarbon and filling of reservoir generally happen just well before compaction. In fact, compation and cementation can decrease the porosity in rocks near reservoir after finishing the migration of non-economic hydrocarbon.