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Net pay cut-off values are key parameters in the evaluation of both conventional and unconventional reservoirs. However, there is no widely accepted method for determining net pay cut-off values. The low permeability turbidite reservoirs in the middle part of the third member of the Shahejie Formation (Es3z) in the Dongying Sag are taken as an example; the net pay permeability and porosity cut-off values are determined according to detailed core descriptions, thin section analyses, porosity and permeability test, mercury injection analyses, and well test data. Taking economic benefits into consideration, the concept of a net pay permeability cut-off value refers to the minimum permeability of a clastic reservoir that can produce hydrocarbons and meet the minimum economic benefits under the availability of existing technologies. The proposed methodology of this work is based on balancing the cost and income, and considers related parameters such as the cost of well drilling, investment in surface construction, operational cost, oil price, and oil production. The net pay permeability cut-off values of reservoirs with different thicknesses of development intervals are calculated based on the constraint of the oil price at $50/bbl. Then, the net pay porosity cut-off values are calculated following the pore-throat structure analyses. The proposed method and the net pay permeability and porosity cut-off values for the low permeability reservoirs in this article are acceptable since fit for purpose principle and data available principle have been considered. Meanwhile, this method enables the dynamic evaluation of reservoirs at any burial depth. • The concept of net-pay permeability cutoff in low permeability reservoirs. • The determination method of net-pay permeability cutoff values. • The determination method of net-pay porosity cutoff values. [ABSTRACT FROM AUTHOR]

The lacustrine deep-water gravity-flow sandstone reservoirs in the third member of the Shahejie Formation are the main exploration target for hydrocarbons in the Dongying Sag, Eastern China. Carbonate cementation is responsible for much of the porosity and permeability reduction in the lacustrine deep-water gravity-flow sandstone reservoirs. The sandstones are mainly lithic arkose with an average framework composition of Q 43 F 33 L 24 . The carbonate cements are dominated by calcite, ferroan calcite, ankerite and a small amount of dolomite. The calcite and ferroan calcite are mainly poikilotopic blocky crystals, while the dolomite and ankerite are mainly euhedral rhombohedra crystals filling intergranular and intragranular pores. The relatively positive δ 13 C values (−2‰ to +3.9‰) of the carbonate cements in the sandstone reflect a mainly inorganically sourced carbon. From 32 Ma to 25 Ma, the pore water was rich in bicarbonate and Ca 2+ due to carbonate dissolution in mudstone, and which were transported with the pore water from mudstone to sandstone via advection and precipitated calcite cementation in thinly bedded sandstones and some high permeability zones in the middle of medium-to-thick sandstone beds. From 12 Ma to present, abundant Ca 2+ , Fe 3+ , Fe 2+ , Mg 2+ and bicarbonate had been transported from mudstone to sandstone via diffusion to form tight ferroan calcite cementation in the upper and lower parts of the medium-to-thick bedded sandstones. Ankerite is mainly distributed in the reservoirs associated with oil migration or charge, because change of Fe 3+ to Fe 2+ from oil charge may supply sufficient Fe 2+ for ankerite precipitation. The center of sandstone beds (>0.6 m) is with potential of high-quality reservoirs in the research area. Carbonate cementation appears to be an important factor that controls the accumulation of oil in deep-water gravity-flow sandstone reservoirs in the study area. [ABSTRACT FROM AUTHOR]

The Eocene sandstones in the northern Dongying Sag, Bohai Bay Basin, China, are reservoirs for large accumulations of hydrocarbons. The sandstones are mainly lithic arkoses and feldspathic litharenites, texturally and compositionally immature. These sandstones have a wide range of porosity (0.4–37%) and permeability (0.004–6969 mD) and show an overall decrease in reservoir quality from 1500 m to 5000 m below sea level. The reduction in reservoir quality is a product of several digenetic processes; these include compaction, precipitation of dolomite and calcite in eodiagenetic stage; compaction, feldspar dissolution, precipitation of quartz cements and clays (kaolin and illite) and precipitation of ferrocalcite and ankerite in mesodiagenetic stage. Mineral distribution pattern and isotopic composition suggest carbonate cements in sandstones originate from sources outside the sandstones. Carbonate cementation, together with compaction reduced the sandstones’ porosity and permeability significantly. In a sandstone bed, marginal sandstones with distance to sandstone/mudstone interface less than one meter always have lower porosity than central sandstones. As burial depth exceeds 4000 m, marginal sandstones have very low porosities (<5%), indicating that thin sandstone beds (<2 m) were totally destroyed by cementation and compaction, and only thick sandstone beds (>2 m) can be potential effective reservoirs. Feldspar dissolution and precipitation of clays and quartz cements have little impact on absolute porosity. Mineral distribution pattern and quantitative data show that leached feldspars are the internal source of authigenic quartz and clays in sandstones, and the volume difference between feldspar secondary porosity and related authigenic cements is generally less than 0.25%. However, although there is little or no net import of matter to the sandstones, the pore architecture changes dramatically. Primary macropores are lost as clays and quartz precipitate while the proportion of microporosity increases, occurring mainly between clay crystals. The overall result is that permeability is significantly degraded. [ABSTRACT FROM AUTHOR]