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Contrary to widely used traditional sequence‐stratigraphic models, highstand sublacustrine fans developed in the deep‐water environment in the middle of the third member of the Eocene Shahejie Formation (Es3z, 42–40.5 Ma), Dongying Sag. Using four cores and 3D seismic data, we describe the sequence stratigraphy frameworks, sediment characteristics, depositional elements, and depositional model of the Liangjialou highstand sublacustrine fan. The Es3z is a complete third‐order sequence stratigraphic, bounded by T4 at the top and T6 at the base. Between T4 and T6, there is the maximum flooding surface (MFS) distributed from proximal to distal of the system in the third‐order stratigraphic sequence. Therefore, the Liangjialou Fan at the top of the Es3z, between MFS and T4, is a typical highstand sublacustrine fan as determined from seismic data. This highstand sublacustrine fan is characterised by gravel to fine‐grained sand, abundant amalgamation surfaces, and crude stratifications common in coarse‐grained structureless‐ to normal‐ graded sandstones. Climbing ripples and thin laminae rich in plant debris are common in inverse‐then normal‐graded sandstones. Channel‐levee, channel‐lobe transition zone, and lobes developed in this highstand sublacustrine fan, with clear internal progradation stacking patterns caused by hyperpycnal flow. The sudden increase in sediment supply in an accommodation decrease situation and the narrow shelf width are the primary controlling factors for forming highstand sublacustrine fans. In the late stage of the highstand systems tract, gravity‐flow deposits caused by hyperpycnal flow are developed from another new point source to form this highstand sublacustrine fan. Hyperpycnal flow in lacustrine basins can transport coarse‐grained sediments with the help of strong flood energy and steep slopes. Gravity flow deposits in lacustrine basins are not confined by sequence stratigraphy frameworks, and many additional areas of sand caused by hyperpycnal flow could be found in lacustrine basins. [ABSTRACT FROM AUTHOR]

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 diagenetic evolution and chemical changes of mudstones have a significant influence on the reservoir quality of the mudstones and their interbedded sandstones. To investigate, a variety of methods were applied to the mudstones in the middle of the third member of the Shahejie Formation (Es3z), which were formed in a lacustrine basin during 43.7–38.2 Ma. Optical microscopy and XRD analysis show that the mudstones are dominated by clay minerals (56.8%) followed by carbonate (18.2%), quartz (14.9%), feldspar (8.4%), pyrite (0.9%), anhydrite (0.8%), and total organic carbon (TOC: 2.4%). Cementation and replacement are the main diagenetic events in the mudstone of Es3z. Carbonates and clay minerals are the most common cements, but occasionally authigenic pyrite, albite, and quartz are also present. At the eogenetic stage, the diagenetic events comprised precipitation of pyrite, siderite, dolomite, and calcite besides transformation of K-feldspar into kaolinite. During the mesogenetic stage, the main diagenetic events included precipitation of fracture-filling calcite, ferroan calcite, ankerite, microcrystalline quartz, quartz overgrowth, and transformation of K-feldspar into illite and albite. The limited variation of the Nb/Ta and Zr/Hf ratios and the almost identical distribution pattern of the REE and trace elements indicate that the detrital material in the mudstones was sourced by a common provenance of felsic igneous rocks. The CIA values of Es3z mudstones vary from 50.59 to 65.05 and the PIA values vary from 50.86 to 75.58, which confirm a low-intensity weathering of the source area. Thus, the variation in the bulk-rock geochemical composition of the mudstones may not be primary, but instead a result of mass transfer with the interbedded sandstones during diagenesis. Changes ratios between aim elements (Al, Si, Ca and K) and immobile elements with burial depth in combination with mass balance calculation indicate that aluminum and silicon were retained by the precipitation of authigenic minerals in the mudstone. The mudstones have received progressively more potassium with increasing depths, whereas calcium has been expelled. The reservoir quality of the Es3z sandstones has mainly been affected by the diagenetically induced decrease in the carbonate content in the mudstones causing carbonate cementation in the sandstones. [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]

Authigenic quartz is a significant cause of reduced porosity and permeability in the cement of the Permian Upper Shihezi Formation (P2s) sandstones in the Gaoqing area, Dongying Sag, Bohai Bay Basin, China. The sandstones are mainly quartzarenite with minor amounts of clay minerals. Authigenic quartz is generally the most commonly occurring cement and occurs as quartz overgrowth, microfracture-healing quartz, pore-filling quartz, and microquartz in the sandstones. Three distinct, separate phases of authigenic quartz (I, II, and III) were distinguished by using cathodoluminescence microscopy (CL). In the scanned CL images, phase I quartz generally appears with light brown luminescence in the 450–670 nm range, phase II quartz is non-luminescent in the 420–480 nm range, and phase III quartz appears with dark brown luminescence in the 470–650 nm range. Phase I quartz, originating mainly from pressure solution, has a composition similar to that of detrital quartz grains and was formed at approximately 79.3–104 °C. In the phase II quartz, Li+, Na+, Mg2+, Al3+, and Ba2+ are enriched, and the homogenisation temperature of some aqueous inclusions is significantly higher than its maximum burial temperature of 130 °C. The δEu of phase II quartz is up to 1.42. The silica sources for phase II quartz are dominated by hydrothermal fluids; pressure solution, feldspar dissolution, and illitisation of smectite also provided a small amount of silica. In the phase III quartz, all of the aqueous inclusions are almost higher than its maximum burial temperature. Al3+, Mg2+, and Rb+ are enriched in most of this quartz, which was sourced mainly from hydrothermal fluids; pressure solution also provided a very small amount of silica. The hydrothermal fluids providing the silica source for phases II and III quartz migrated into the P2s sandstones along the adjacent Gaoqing Fault. • The siliceous sources are dominated by external hydrothermal fluid and internal pressure solution of quartz grains. • Hydrothermal fluid was proved by the homogenisation temperature and the chemical composition of mineral assemblages. • Three distinct, separate phases of authigenic quartz were distinguished. • Phase I quartz is mainly from pressure solution. Phase II and III quartz are dominated by deep hydrothermal fluids. [ABSTRACT FROM AUTHOR]