The Burial of Carbonate Reservoirs: “The rest of the story”

Mateu Esteban, REPSOL-YPF, Madrid, Spain, with co-author, Conxita Taberner, SHELL SIEP/EPT-SCRM, Rijswijk, The Netherlands

Reservoir properties are related to, or modified by: (i) primary depositional environments, (ii) early diagenesis at or near depositional surface, at shallow burial under the influence of surface-controlled processes, (iii) deep burial diagenesis away from surface-controlled processes. The present-day reservoir is the balance of porosity creation and destruction after a long diagenetic evolution.

It is commonly assumed that the burial environment is not very active, or that any diagenetic change in the burial environment is essentially controlled by depositional facies patterns or shallow diagenetic imprints. This is not always necessarily the case. In particular the deep burial of carbonate reservoirs deserves more attention. After all hydrocarbon reservoirs and hydrocarbon migration occur in the deep burial environment.

The burial of carbonate reservoirs shows a common pattern with the following stages:

A. Standard carbonate cementation sequence, with:

1. increasing compaction and pressure dissolution,
2. several minor discontinuities between cement stages,
3. increase of temperature as recorded in fluid inclusion microthermometry.

Fracture and inherited early porosities are extensively obliterated, although some may locally survive into later diagenetic stages.

B. Local thermobaric (“hydrothermal”) alteration, with sudden influx of higher temperature, fracture-fed fluids associated with hydraulic brecciation, cessation of pressure dissolution and re-opening of stylolites. Any remnant of early porosity might be locally enlarged, but porosity is mostly redistributed or obliterated. Traces of migrated hydrocarbons might be locally detected in the cements of Stage B. Two different pathways are observed at this stage, leading to coarse dolomite replacement or massive calcite cementation.

C. Generalized corrosion of previous fabrics, interpreted as result of mixing and/or cooling of brines and local inflow of organically-derived CO2 associated to hydraulic fracturing. Precipitation of minor amounts of quartz, dickite, fluorite, feldspar, sulfides and celestite is characteristic; traces of MVT minerals are locally present. Major hydrocarbon charge occurs at this stage. Two different pathways are observed:

1. locally abundant cementation by late poikilotopic calcite or sulfates
2. minor calcite/sulfate cementation and major porosity development.

The diagenetic Stage C is considered as the most critical for the overall improvement of reservoir properties. Potential controlling factors appear to be: proximity to major faults and fault zones, location of fluid contacts and source rock kitchen areas, location of evaporitic units, burial history and late structural evolution. In addition, diagenetic pathways in Stage C are largely controlled by previous burial history and lithologies. Late poikilotopic calcite cements appear as related to Ca-rich residual fluids after Stage B dolomitization, and to the significant inflow of organically-derived CO2.

In general, the thermobaric Stage B does not necessarily improve porosity or reservoir connectivity; in fact, it appears to be absent or poorly developed in many excellent reservoirs where porosity and connectivity was mostly related to Stage C. When present, Stage B may control the development of Stage C (i.e.: mechanical discontinuities, brittleness, conduits/barriers). Basin evolution, thermal gradients, burial depths and host rock lithologies are considered amongst the main controlling factors.

Strategies for understanding and predicting the distribution of present-day reservoir properties largely depend on the level of control of depositional facies and early diagenesis on late burial diagenetic processes. This requires integration of structural evolution, basin analysis, sequence stratigraphy, seismic attribute analysis, petrophysics, advanced reservoir petrology, cement stratigraphy and fluid flow and water:rock interaction models. The burial of the reservoir is here perceived as the most critical part to be understood in the reservoir history.