Geomorphology, lithofacies and sedimentary environment of lacustrine carbonates in the Eocene Dongying Depression, Bohai Bay Basin, China

Marine and Petroleum Geology 113 (2020) 104125

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Research paper

Geomorphology, lithofacies and sedimentary environment of lacustrine carbonates in the Eocene Dongying Depression, Bohai Bay Basin, China

T

Shengqian Liua,b,c,∗, Zaixing Jiangd, Youbin Hea,c,∗∗, Luxing Doue,∗∗∗, Yang Yangd, Yang Lid, Chao Hanb a

Key Laboratory of Exploration Technologies for Oil and Gas Resources (Yangtze University), Ministry of Education, Wuhan, 430100, China Shandong Key Laboratory of Depositional Mineralization & Sedimentary Mineral, Shandong University of Science and Technology, Qingdao, 266590, China c College of Geosciences, Yangtze University, Wuhan, 430100, China d School of Energy Resources, China University of Geosciences, Beijing, 100083, China e College of Geosciences, China University of Petroleum, Beijing, 102249, China b

ARTICLE INFO

ABSTRACT

Keywords: Lacustrine carbonate platform Geomorphology Facies model Sea transgression Dongying Depression

The Eocene lacustrine carbonates in the western Dongying Depression, Bohai Bay Basin were examined based on integrated analysis of 3D seismic data, cores, thin-sections, and geochemical testing. Seismic profiles and geomorphological mapping revealed a landmass-attached (semi-isolated) carbonate platform restricted by marginal syn-sedimentary faults and surrounded by three deep sags. 3D topographic analysis of the platform revealed six districts characterized by variable buried depths, topographic gradient, and fault combinations, including two flat-topped areas (PFW and SD; topographic gradient < 4°) and four transitional zones above deep sags (P1, P2, S1, and S2; 4–10°). Eight lithofacies types were identified, forming five lithofacies associations: reef-shoal complex deposits, small bio-buildups, shoal-intershoal deposits, event flow deposits, and near-landmass mixed deposits. Well cross-sections revealed that facies distributions were closely associated with the geomorphological districts. Therefore, a geomorphology-dominated carbonate platform model was constructed, which highlighted the relationship of platform topography, hydrodynamic conditions, and facies distributions. The littoral facies is mainly composed of medium–high-energy deposits including reef-shoal and shoal sediments, which are mostly distributed in flat-topped areas. The sublittoral facies is dominated by low-to medium-energy deposits but influenced by storm activities, especially in the windward step-fault transitional zones. The profundal facies is positioned away from the platform marginal faults and joined with surrounding deep sags. Besides the regional geomorphology and its controlled hydrodynamic zones, the formation of such considerable lacustrine carbonates throughout the whole basin may have occurred because the global warming of late Eocene period induced sea transgression, which facilitated the development of lacustrine carbonates via marine-sourced reef-builders and biologically-induced carbonate precipitation. Furthermore, the East-Asian paleomonsoon climate influenced regional windfield may also have enhanced the prevailing southeast wind, which influenced the distribution of lithofacies and favorable reservoirs. Porosity-permeability analysis revealed that reservoir properties have a close relationship with geomorphology and lithofacies, thus geomorphology-associated facies prediction is essential for further hydrocarbon prospecting in this area.

1. Introduction Lacustrine carbonates are universally present in continental basins, and in recent years they have attracted increasing attention as important oil and gas reservoirs (Muniz and Bosence, 2018; Peng, 2011;

Thompson et al., 2015; Yang et al., 2017) or important indicators for paleoenvironment or paleoclimate (Arp, 2006; Frantz et al., 2014; Lettéron et al., 2017, 2018; Mizuno et al., 2018; Sarg et al., 2013). However, lacustrine carbonates are usually relatively smaller in volume and less well studied than marine carbonates (Burton et al., 2014;

∗ Corresponding author. Key Laboratory of Exploration Technologies for Oil and Gas Resources (Yangtze University), Ministry of Education, Wuhan, 430100, China. ∗∗ Corresponding author. College of Geosciences, Yangtze University, Wuhan, 430100, China. ∗∗∗ Corresponding author. College of Geosciences, China University of Petroleum, Beijing, 102249, China. E-mail addresses: [email protected] (S. Liu), [email protected] (Y. He), [email protected] (L. Dou).

https://doi.org/10.1016/j.marpetgeo.2019.104125 Received 24 June 2019; Received in revised form 31 October 2019; Accepted 4 November 2019 Available online 07 November 2019 0264-8172/ © 2019 Elsevier Ltd. All rights reserved.

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Gierlowski-Kordesch, 2010; Platt and Wright, 1991), and facies models for lacustrine carbonates and the understanding of formation mechanism are scarce and insufficient (Muniz and Bosence, 2018). Lacustrine carbonates of the upper 4th member of Shahejie Formation (Es4s) are well developed in the Eocene Dongying Depression, and especially accumulated in paleotectonic highs. On some structural highlands (e.g. the Pingfangwang high), the green algal Cladosiphonia were commonly discovered (Zhu, 1979) and considered as main contributors for lacustrine reef buildups (Du, 1990; Qian and Wang, 1986). Nevertheless, previous studies have mostly concentrated on the overall feature of these areas and depicted them as underwater highs for lacustrine carbonates or large-scale reefal basements (Du, 1990; Jiang, 2011, 2018; Li et al., 2006). In fact, lacustrine carbonates are usually characterized by frequent alterations and rapid evolution vertically and laterally (Sarg et al., 2013; Seard et al., 2013; Tanavsuumilkeviciene et al., 2017), which is greatly different with marine carbonates. Furthermore, the deposition of lacustrine carbonates is sensitive to various environmental factors, including tectonic, climate, siliciclastic input, and hydrology, which significantly influence the formation of lacustrine carbonates (Platt and Wright, 1991; Wang et al., 1993; Yang et al., 2017). However, lacustrine carbonates in the Eocene Dongying Depression are mostly described and documented sketchily (Du, 1990; Li et al., 2006; Wang et al., 1993; Zhou and Du, 1986), lacking detailed analysis of facies distribution, depositional model, and intrinsic controlling factors. Further investigations on geomorphology, hydrodynamics, and paleoclimate are also insufficient. Hence, additional research is needed to promote the understanding of lacustrine carbonates. In this study, the detailed geomorphological districts were introduced based on 3D seismic data, and geomorphology- and lithofacies-based paleoenvironment analyses were carried out based on newly-observed sedimentological data (cores, thin-sections, loggings, and rare earth elements). These investigations well improve our understanding of hydrodynamic conditions, the depositional model, and the favorable factors for lacustrine carbonate formation in the Dongying Depression. The overall purposes of this study were: 1) to provide an alternative lacustrine carbonate depositional model; and 2) to enhance present knowledge of the formation mechanism of lacustrine carbonates regionally and globally. The results of this study will facilitate analysis of the sedimentary system, origins of these lacustrine carbonates, and future prediction of favorable reservoirs in rifting lacustrine basins.

(bottom unconformity), submerged during the TST and HST periods, and exposed again during the early Es3 period (Fig. 2B). The overall sedimentary setting for carbonate deposition was a carbonate platform located far away from the major siliciclastic sedimentary system (Du, 1990; Jiang, 2018). 3. Methods This study was mainly based on seismic, core, and well logging data, alongside thin-section observations and geochemical testing data. The 3D seismic data cover an area of approximately 600 km2, including the buried hill and its adjacent area in the western Dongying Depression. The regional strata interpretation results and regional time-depth conversion table were supplied by the Geological Institute of the Shengli Oilfield, Sinopec. The overall geomorphology was based on seismic profiles and revealed continued subsidence. Thus, the current geomorphology should have mostly inherited the original platform paleogeomorphology. The morphology analyzed in this paper was based on the seismic interpreted reflector T6 after a calculation from time to depth (Fig. 4A). Furthermore, digital elevation modeling and 3D slope analysis (Fig. 4B) were realized using ArcGIS 10.2 software (ESRI, Inc., CA, USA). Well logging data from 100 wells were collected, approximately 300-m-long cores from 13 wells were observed. 220 samples were systematically selected (one sample every 1–1.5 m by core-drilled depth) to cover all lithofacies and cut into thin-sections for microscopic petrographic studies, and then alizarin Red-S was applied to one third of the area of every thin section for differentiation of calcite and dolomite. In addition, partial reservoir samples were impregnated with blue resin and then cut into thin sections to observe pores under the microscope. The major carbonate grains and their origin and diagnostic criteria were followed the terminology defined by Flügel (2010), and lithofacies identification was based on the modified Dunham classification by Wright (1992). Lithofacies associations were constructed and correlated across the platform to determine the depositional environment and facies. 4. Results 4.1. Seismic geomorphology The seismic profiles crossing the study area indicated that the general geomorphology is a structural highland limited by large-scale marginal normal faults (Fig. 3). The topographic map shows that the highland is surrounded by three deep sags and adjacent with the northern Linfanjia landmass, thus forming a semi-isolated platform (Fig. 4A). The slope gradient map shows distinctly marked marginal faults with large inclination degrees (Fig. 4B). Thus, platform margins were drawn in combination with the morphology, the distribution of slopes and their correlation with faults (Fig. 4). There are two denudation areas on the platform top zone (Fig. 4A), resulting from the tectonic uplifting after the Es4s deposits (Jiang, 2011; Li et al., 2006). Nevertheless, the carbonate intervals remain continuous in most areas across the platform (Fig. 3), which indicates that the platform did not experience distinct geomorphological transformations after the Es4s period. In general, the present topography could be considered to have been inherited from the structural paleogeomorphology contemporary with the Es4s carbonate deposits. Multistep-fault zones were observed between the platform top areas and marginal faults, which could be caused by contemporary rifting and subsiding because most faults do not cut through the T6 surface (Fig. 3A and B). These step-fault zones are characterized by larger inclination degrees than the top areas (Fig. 4B), which presented as transitional zones from platform to basin. In the NW–SE oriented sections (Fig. 3A), the platform contains two sub-highs, Shangdian (SD) and Pingfangwang (PFW). There are

2. Geological setting The Dongying Depression is located in the southeast corner of the Bohai Bay Basin of China (Fig. 1A). It is a Cenozoic rift sub-basin inherited from the late Cretaceous extensional tectonic background. The study area presents a low buried-hill draping structural belt and marks the western margins of the Dongying Depression (Fig. 1B). This buried hill was a fault-uplifted highland during Eocene period, surrounded by three sags (Lijin sag, Boxing sag, and Lize sub-sag) and bordered by large synsedimentary faults (Fig. 1C). The strata in Dongying Depression consist of Paleogene Kongdian (Ek), Shahejie (Es), and Dongying (Ed) Formations; and Neogene Guantao (Ng) and Minghuazhen (Em) Formations (Fig. 2A). The Shahejie Formation (Es) is subdivided into four members, of which lacustrine carbonate rocks are more developed in upper sections of the upper fourth sub-member (Es4s). The Es4s interval corresponds to a thirdorder sequence, between the reflectors T6 and T7 in seismic profiles (Fig. 3). Carbonate rocks are mainly accumulated in the uppermost highstand system tracts (HST), including the lower parasequence set (PSS) H-1 and the upper H-2 (Fig. 2B), which are comparable throughout the whole basin (Jiang, 2018). The studied carbonate deposits on the western marginal highland belong to an incomplete 3rdorder sequence, which was mostly exposed during the LST period 2

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Fig. 1. Location, tectonic setting, and structural map of the Dongying Depression. (A) Tectonic setting of the Bohai Bay basin showing the pull apart feature. (B) Structural map of the Dongying Depression. (C) Schematic structural map of the study area, with key synsedimentary faults bordering the platform. Sections A–A′ and B–B′ indicated by dotted lines in (C) are the locations of the cross-sections presented in Fig. 3A and B.

topographically flat highs on the platform, with slope inclination degrees generally less than 4° (Fig. 4A and B). Between the two highs is a topographic low area controlled mainly by the synsedimentary faults F3 and F4 (Fig. 1C). The low area becomes wider and deeper to the west, joining to the Lize sub-sag; and to the east, it becomes narrower and shallower, transferring into a flat-topped zone where PFW and SD join together. In the W–E oriented sections (Fig. 3B), the platform shows an anticline-like topography with a high flat-topped zone in the middle and two basinward-inclined transition zones on the flanks. Six districts are recognized based on slope gradient and internal morphology distribution: SD high, S1, and S2 in the north part; and PFW high, P1, and P2 in the south part (Fig. 4A). For the northern SD high, most areas are denuded and connected with the Linfanjia bulge. The preserved high flat-topped area is nearly E–W oriented, consistent with the strike of fault F3 (Fig. 4A). District S1 presents a W–E trending belt located in the south of the SD high, which is narrower and steeper in the east part than the west. District S2 is a very narrow belt restricted by the denudation area and fault F5, preserved as a small multi-step-fault zone. Both S1 and S2 are predominantly limited by platform marginal faults, and act as transitional zones from shallow platform to deep waters. The southern PFW high is a flat-topped zone oriented between NE–SW and N–S. Its south margin is cut down by the synsedimentary deep fault F1, almost adjacent to Boxing sags separated by a relatively narrow transitional zone. In contrast, the west and east of the PFW high

are two wide transition zones, western district P1 and the eastern district P2 (Fig. 4A). Both P1 and P2 are preserved as multi-step-fault zones with a general topographic gradient ranging from 4° to 10° (Figs. 3B and 4B), suggesting synsedimentary gentle-slope zones above the platform. 4.2. Lithofacies types Eight main lithofacies types were recognized (Table 1) using integrated macro- and micro-fabrics based on the modified Dunham classification of carbonates by Wright (1992). Determination of the sedimentary environment of Es4s carbonates was based on an integrated analysis consulting the interpretations of Arp (1995) and Sarg et al. (2013). 4.2.1. LF1: framestone, bafflestone LF1 predominantly occurred on the flat-topped areas in the SD high, PFW high, and upper parts of P2. Framestone and bafflestone, in the study area, are often too unconsolidated and vuggy to be drilled as complete cores. The well-preserved cores are generally well cemented, with porosity partly to totally sparite-filled. Microscopically, LF1 is characterized by the tube-like green algae named Cladosiphonia (Du, 1990, Fig. 5A–C). These tubes are fibrous and fragile with diameters generally ranging from 30 to 60 μm. Their tube walls are mostly composed of micrite and cemented with rimmed dolomite (Fig. 5A), 3

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Fig. 2. (A) General stratigraphy of the Dongying Depression, Bohai Bay Basin (modified from Feng et al., 2013) and (B) Es4s sequence stratigraphy in the western Dongying depression.

forming a rigid aggregate fabric. This fabric has well preserved the original algal skeleton and displayed very good interparticle porosity contributing to good reservoir properties. Abundant peloids are deposited among these tubes, and undamaged ostracods are also common (Fig. 5A–B). These baffled grains show moderate to well sorting and abrasion. In addition, the thickness of tube walls is generally about 5–15 μm and even have branches (Fig. 5C) that are easily broken apart and preserved as algal fragments when weakly cemented. Interparticle porosities are commonly observed in these broken algal tubes and fragments. Interpretation: The bafflestone fabric suggests high hydrodynamic energy, as evidenced by the growth of abundant green algae, named Cladosiphonia sinensis and Cladosiphonia shandongenis (Zhu, 1979). These green algae are commonly distributed in shallow sea of subtropical to tropical areas shallower than 12 m (Qian and Wang, 1986). This type of green algae was also similar to the Cladophorites framestone and bafflestone discovered as algae bioherms in the reef belt of the Ries impact crater in southern Germany, which was interpreted as a eulittoral to supralittoral environment controlled by seasonal changes of lake level (microfacies MF26 and MF27, Arp, 1995). The well-developed sparite-cemented porosity in LF1 indicates an agitated

depositional environment and highly hydrodynamic conditions. Considering that the Eocene Dongying lake was strongly influenced by wind–waves (Jiang, 2018), the fragile lithofacies LF1 are hard to accumulate large solid wave-resistant buildups. In addition, lithofacies LF1 are vertically and laterally limited in distribution, thus LF1 was interpreted as small scattered patchy reefs deposited in the upper to middle littoral zone (i.e. upper to middle shoreface). 4.2.2. LF2: boundstone Lithofacies LF2 occurs not only in the flat-topped areas where LF1 develops, but also in parts of the transitional zones P2, S1, and S2. Its presence in District P1 is not clear because there were no cored wells for this district. Two main sub-types with different fabrics were recognized as thrombolite (Fig. 5D) and stromatolite (Fig. 5E–F), namely LF2.1 and LF2.2, respectively. LF2.1 consists of multiperiodic accreted micrites, or aggregates of clotted micrite with peloids, bio-clasts, and coated grains. The skeleton and interparticle pores are dominated pore types for reservoir properties. The rims of thrombolite are abraded with abundant peloids scattered outside, which are generally moderately to highly abraded, moderately sorted, and sparite-cemented (Fig. 5D). Within the clotted micrite, multiple periodic accretions were observed. 4

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Fig. 3. Seismic profiles of the (A) A–A′ and (B) B–B′ sections showing topography of the platform. Section locations of A–A′ and B–B′ are shown in Fig. 1C.

LF2.2 is characterized by laminated structures, composed of millimeter-thick concordant laminas, and various laminae growth shapes were observed, including flat planar, undulating, and small columnar. The flat to undulating stromatolites (LF2.2A) are generally thinner in lamina thickness and underlain or overlain by LF4 or LF5, whereas the small columnar ones (LF2.2B) contain thicker lamina and are accompanied by LF3 or LF4 (Fig. 5F and G). In particular, considerable amounts of oncoid or peloid grains are present among the frameworks of these anastomosed branching stromatolites, which are closely packed, completely abraded, and well-sorted (Fig. 5G). In comparison, LF2.2A is often of poor porosity, whereas LF2.2B associated fabrics generally have excellent intergranular porosity. Interpretation: The abraded thrombolite (LF2.1) and its associated matrix-absent, sparite-cemented peloidal grainstone could be interpreted as littoral deposits in middle-high hydrodynamic conditions, in which those peloids or coated grains are originally derived from the clotted micrite. For stromatolite (LF2.2), the laminated nature indicates a balanced condition between microbial growth and sediment supply (Reid et al., 2000), and the range of growth patterns could be related to different ecological and sedimentological conditions (Sarg et al., 2013). Similar to LF1, the small columnar and branching stromatolites (LF2.2B) capped or associated with grainstones (Fig. 5F and G) could reflect an agitated water environment. These stromatolites, as well as the associated thrombolite and grainstone, are widely distributed throughout the most flat-topped area to the upper part of transitional zone S2 and P2, and probably contributed to the development of scattered reefs or small bio-buildups in littoral to sublittoral zones

throughout the platform. In contrast, the flat to undulating laminated stromatolites (LF2.2A) are commonly interbedded with wackstone to calcimudstone and mainly distributed in the transitional zones, which may suggest a sublittoral environment characterized by a relatively low hydrodynamic energy (above the storm wave base, in the photic zone). 4.2.3. LF3: intraclastic, oolitic, peloidal grainstone LF3 occurs predominantly in the flat-topped areas of the platform. On cores, it shows yellow to greyish, and is lithologically closely associated with LF1, LF2, and LF4. Microscopically, this lithofacies mainly consists of densely packed or sparite-cemented grains, including intraclasts, peloids, ooids, and bioclasts (Fig. 6). Interparticle, intraparticle and moldic porosities are dominant and display excellent reservoir properties. Generally, distinct boundaries were observed between lithofacies LF1 and LF2 and their associated grainstone (named LF3.1, Fig. 6A–D). The grainstone contains abundant intraclasts such as clotted micrite fragments (Fig. 6B) and algae-tube debris of Cladosiphonia (Fig. 6C), showing random stacking, poor roundness, poor sorting, and slight abrasion. Moderately to completely abraded intraclasts were also observed in local areas, showing various degrees of sorting and abrasion (Fig. 6C). In addition, the grainstone associated with bafflestone often contains micrite-cemented peloids and ostracods, in which unbroken ostracods and slight abraded peloids were commonly observed (Fig. 6D). However, those that did not associate with lithofacies LF1 and LF2 had grains that appeared more closely-packed with better roundness and sorting (named LF3.2, Fig. 6E and F). The most common components of LF3.2 are sand-size peloids and ooids 5

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Fig. 4. (A) Buried depth map of seismic reflection T6, showing a semi-isolated platform geomorphology surrounded by three sags. Six topographic districts are marked, namely PFW, P1, P2, SD, S1, and S2, see text for details. (B) Slope gradient distribution map. The boundaries of platform margins and flattened tops were drawn mainly based on general geomorphology, marginal faults, and inclination degrees.

(> 80% of grain content), as well as some gastropod and ostracod fragments (generally not exceeding 20% of grain content). Ooids are the dominant coated grains in the study area, most of which have sandsize micrite nuclei and thin cortex and some ooid nucleus are ostracod or gastropod fragments (Fig. 6F). Interpretation: The slight abrasion and poor sorting of clotted micrite or algal fragments are originally homogenous with the in situ boundstone or bafflestone, which indicate very short-distance transportation. Considering the associated Cladosiphonia or thrombolite, LF3.1 is interpreted to be in situ sediments derived from reef buildups due to water agitation in highly hydrodynamic settings, and then deposited at reef

tops or fringes. The intergranular cavities are matrix-free or spariteinfilled, reflecting strong wave-winnowing in this high-energy environment, whilst the relatively well-preserved ostracods in this lithofacies could represent a less-agitated and protected microenvironment among reef buildups. In contrast, LF3.2 was interpreted as shoal deposits developed in areas without reef buildups, considering its independence to the reef-associated lithofacies. Furthermore, the general moderate-to-well-sorted, rounded and matrix-free grainstone was attributed to the strongly wave-winnowed environment.

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Ripple bedding, siliciclastic and marly fragments. Yellowish to grayish

Moderate- to high-energy environment, upper littoral.

Laminated or massive, fine quartz grains and stem fragments commonly observed.

4.2.5. LF5: intraclastic, bioclastic wackestone LF5 is distributed in most parts of the platform, with an increasing trend developed from flat-topped areas to step-fault zones. In cores, the lithofacies is generally grayish to gray or brown and frequently interbedded with calcimudstone. It appears thinly laminated, with small wavy to horizontal beddings commonly observed. Thin-section observation revealed that lithofacies LF5 are dominantly composed of light-color intraclasts and bioclasts and dark-color micrite matrix (Fig. 7C–F). The intraclasts are characterized by scattered lime-mud ripups, which are chaotically distributed or parallel to bedding, without any sorting and roundness (Fig. 7C). The bioclasts consist of ostracod and gastropod clasts, which are mostly fragmented, flattened, and parallel to the bedding (Fig. 7D, F). Porosity is poor for this lithofacies, some shells are dissolved but their moulds are filled by calcite cement. In the vertical direction, the intraclastic-bioclastic wackestone alternates with calcimudstone or packstone, and sharp-contacted surfaces are commonly present between them (Fig. 7E and F). Microscopically, abundant flattened or wavy scour structures occur with the frequent alterations and the particles become finer in the upward direction, forming stacked fining-upward sequences (Fig. 7E and F). Interpretation: The matrix-supported fabric, as well as the commonly observed horizontal bedding and dark color, suggests a generally lowenergy environment with little hydrodynamic activity. The associated small wavy or undulant beddings reflect weak wave influences that occurred near the wave base, perhaps in lower littoral to sublittoral zones. Considering that the lithofacies is distributed more often in the step-fault zones (e.g. Wells B432c, B414, B182, Fig. 4) and storm activities were frequent in the study area (Jiang, 2018; Wang et al., 2015), the commonly observed oriented rip-up clasts, ostracod fragments, and associated scouring structures could be interpreted as the products of gravity flows induced by storm events, which usually occur in the sublittoral to the upper profundal zones (Flügel, 2010). 4.2.6. LF6: intraclastic rudstone to floatstone Lithofacies LF6 occurs mainly in margins of flatted high areas and step-faulted transition zones. This lithofacies consists of two subfacies, rudstone and floatstone, which are both poorly sorted with a wide grain size ranging from 0.1 to 7 cm. Rudstones were more commonly observed on the southern margins of flat-top areas to the upper part of transitional zones, whereas floatstones more often occur on the lower part of step-fault transitional zones. In the cores, these lithofacies are generally yellowish or grayish to gray, but the gravels are mainly gray to dark (Fig. 8A–C). Most gravels appeared to be non-oriented to slightly oriented, poorly-sorted, and low-to medium-abraded. Moreover, truncated structures were observed between rudstone and

LF8

LF7

LF5 LF6

4.2.4. LF4: intraclastic, peloidal, bioclastic packstone LF4 is usually distributed in flat-topped areas and the upper part of step-fault zones. In cores, the lithofacies is yellowish to greyish, closely associated with LF3 and LF5 with fuzzy planar beddings in the vertical direction. Microscopically, the most commonly observed grains are siltto sand-sized peloids, intraclasts, and bioclasts (Fig. 7A and B). These grains form a packed fabric with largely fine-grained particles as residual matrix filled in interstices. Some interparticle, intraparticle and moldic porosities are observed. It is also common for a few gravel-size grains, e.g. unbroken gastropod shells (Fig. 7A), complete ostracods, and irregular-shaped peloidal grainstone intraclasts (Fig. 7B) to be distributed between these ordinary silt-to sand-sized grains. Interpretation: Lithofacies LF4 was interpreted as deposits in a relatively lower hydrodynamic environment than that of LF3, as evidenced by the packed, poorly-washed fabric and the existence of unbroken shells of gastropods and ostracods (Fig. 7A and B). It probably represents less-agitated and protected intershoal environment in backshoal or lateral lower areas, where the coarse grainstone clasts indicate redeposited grains derived and transported from shallower shoal bodies and eventually deposited in their adjacent lower-energy environment.

Argillaceous limestone to calcimudstone Arenaceous limestone

Yellowish to gray

Low-energy, littoral back-reef or inter-shoal deposits, or sublittoral deposits. Low-energy background, lower littoral to sublittoral above the storm wave base, episodic storm reworking deposits. Low-energy setting. Thin-laminated, wavy to horizontal beddings observed, matrix-supported. Randomly scattered gravels, fining or coarsening upward sequence. Grayish to brown Yellowish to gray

Moderate-energy, littoral, back-reef or intershoal deposits. Grains are poorly sorted and rich in lime matrix, containing unbroken gastropods. LF4

LF2.2: Stromatolite Intraclastic, oolitic, peloidal, grainstone Peloidal, oolitic, bioclastic packstone Intraclastic, bioclastic wackestone Intraclastic rudstone to floatstone LF3

LF2.1: Thrombolite LF2

Yellowish and gray Yellowish to grayish

Yellowish to gray

Moderate- to low-energy, littoral to sub-littoral, small bio-buildups. High-energy, littoral, reef-shoal complex or shoal deposits.

Moderate- to high-energy, littoral to sub-littoral, reef buildups.

Framestone, bafflestone LF1

Yellowish to grayish

Fragmented on cores, microscopically green algal Cladophorites dominated. The calcified algal tubes baffle, trap and bind grains. Nodule-like structure, composed of micrite accretions or aggregate-binding peloids, ostracods etc. Various growth patterns as small columnar, wavy, and flat laminated. Poor- to well-abraded and closely packed grains, bioclast-rich, matrix absent. Yellowish to grayish

Moderate- to high-energy, littoral algal reef buildups.

Sedimentary structures and textures Color Lithofacies and lithology

Table 1 Main lithofacies types of Es4s lacustrine carbonates in the western Dongying Depression.

Interpretation

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Fig. 5. Photomicrographs of lithofacies LF1 (A–C) and LF2 (D–G). (A) Framestone with a tuft of Cladosiphonia tubes (transection), the tube walls consist of micrite and rimmed by isopachous dolomite cement (white arrows), pel. – peloid, ostr. – ostracod; Well 246, 1530.6 m. (B) Bafflestone with tube-shaped green algal Cladosiphonia (white arrows) and trapped mud-lime, peloids, and ostracod carapaces; Well B80, 1451.4 m. (C) Bafflestone with branched Cladosiphonia tubes (yellow arrows) and associated algal fragments and peloids; Well B182, 1632.5 m. (D) Boundstone consisting of clotted micrite with associated peloids (the same origin); Well B80, 1460.3 m. (E) Undulatory stromatolite with a base of clotted micrite; Well B706, 1594.9 m. (F) Small columnar stromatolite with an abraded base and associated sparitic peloidal grainstone; Well B414, 1470.15 m. (G) Anastomosed branching stromatolite, with the enlarged red box part showing peloids and oncoids infilling between the columns; Well B432c, 1569.4 m. The stromatolite shapes were identified with reference to James and Jones (2016). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

common storm activities during the late Es4s period in the study area (Jiang, 2018; Wang et al., 2015), LF6 was interpreted to have been deposited in the lower littoral to sublittoral environment, mostly between the fair-weather wave base and the storm wave base.

abruptly capped laminated lithofacies (LF5; Fig. 8B), and strong scouring structures were commonly observed between different staged floatstones (Fig. 8C). Microscopically, the intraclastic rudstones are composed of gravels and sand-size grains with the same composition, which presents a grain-supported fabric that does not contain much micrite matrix (Fig. 8D). In addition, the gravels also contain varied fragments such as grainstone, wackestone, and micrite (Fig. 8E), some of which are vuggy that interparticle and intraparticle porosities can be seen. In contrast, the floatstones contain more micrite matrix forming micrite-supported fabrics, and unbroken ostracod shells were also commonly observed. Interpretation: The chaotic intraclast orientations, poor sorting, variable sourced fragments, combined with cyclic fining-upward sequences, truncated structures and scouring structures could be interpreted as event flow deposits that may have been induced by storm events (Al-Awwad and Pomar, 2015; Flügel, 2010; Mizuno et al., 2018; Muniz, 2013). The storm scours the unconsolidated deposits and leads intraclastic gravels to be suspended or transported, and settled quickly and chaotically during the storm weakening stage. The dark micrite intraclasts reflect the fragments of pre-deposited calcimudstone derived from parautochthonous deposits, and the grainstone intraclasts may be brought in from shallow shoals by storm backflows. The increasing micrite matrix content, as displayed by the rudstones and floatstones of LF6, indicates decreasing intensity of hydrodynamic conditions from margins of flat-top areas and the upper parts of transitional zones to the lower areas. Considering the overall hydrodynamic conditions and

4.2.7. LF7: argillaceous limestone to calcimudstone Lithofacies LF7 mainly occurs in the adjacent areas of Linfanjia rise and Shangdian denudation, step-fault transitional zones, and surrounding sags beyond platform marginal faults, and is barely developed on the flat-topped areas. In the cores, this lithofacies generally appeared yellowish to gray and thinly laminated, and contained lots of large grain-size fragments of charophyte stems (Fig. 9A). The thin-section observations revealed that the argillaceous limestone usually contains some siliciclastic components (e.g. fine quartz grains, Fig. 9B). On the platform, argillaceous limestone is often associated with mudstone or calcimudstone, forming thin-interbedded laminations. In contrast, in the surrounding sags, it is generally associated with mudstone, showing dark gray to blackish color, thin-laminated bedding, and a lack of fossils. Interpretation: The distribution above marginal faults, light-color appearance, abundant fragments of charophyte stems, and commonlydeveloped horizontal beddings together indicate that the lithofacies LF7 was deposited in a shallow and low-energy deposition environment. This environment corresponds well to the upper littoral zone, which is similar to the lake shore environment where charophycean mudstone was deposited (Flügel, 2010). In contrast, the dark color, thin 8

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Fig. 6. Photomicrographs of lithofacies LF3. (A) Peloidal grainstone associated with the early clotted micrite, separated with micrite crust; Well B246, 1530.6 m. (B) Intraclastic packstone, mainly composed of thrombolite fragments (white arrows); Well B303, 1713.4 m. (C) Loosely packed peloids and intraclasts (white arrow) containing micritic algal tube fragment (yellow arrow); Well B182, 1635.7m. The blue parts are vuggy porosities filled with epoxy resin. (D) Ostracod-rich grainstone, micrite infilling ostracod cavities; Well B76, 1485.0 m. The thin section was stained with Alizarin. (E) Peloidal grainstone, showing peloids with high sorting and roundness; Well B246, 1545.8 m. (F) Oolitic grainstone with peloid, ostracod, or gastropod fragments as ooid nucleus (yellow arrows), most ooids showing thin cortex; Well B182, 1640.55 m. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

lamination, and lack of fossils should reflect sublittoral to profundal deposits or hydrological closed lake conditions.

land littoral environment influenced by waves. 4.3. Lithofacies associations and distribution

4.2.8. LF8: arenaceous limestone Lithofacies LF8 mainly occurs in TST from vertical stratigraphic column and was rarely observed in HST from the observed cores. The cutting logging interpretation results revealed that this lithofacies generally appears as interbedded sandstone-siltstone and limestonemarlstone, and mainly developed near the northern Linfanjia Rise. The cores indicated that this lithofacies appears yellowish and shows asymmetrical wave-ripple bedding (Fig. 9C). It is a mixture of siliciclastic and carbonate components, containing a marked amount of sand-size siliciclastic grains with moderate sorting and abrasion (Fig. 9D). These grains form a packed fabric with lime-mud matrix filled in the interstices. The associated lithofacies are generally siliciclastic rocks such as sandstone, mudstone, and argillaceous limestone. Interpretation: The light color and ripple bedding indicate a waveinfluenced shallow water environment. The high percentage of sandsized quartz grains in the carbonate-dominated areas reflects a nearprovenance setting with high siliciclastic input, which was also indicated by the laterally and vertically associated sandstone or mudstone. Furthermore, the moderate sorting and abrasion, poorly-washed interstices, and wave ripples indicate moderate-energy hydrodynamic conditions. Thus, LF8 was interpreted to have been deposited in a near-

4.3.1. Lithofacies associations Based on identifying individual lithofacies and their stacking patterns, five main lithofacies associations were recognized, with each association representing a distinct facies succession (Fig. 10). The typical LFA1 mainly includes three lithofacies that were deposited under moderate-to high-energy hydrodynamic conditions, recording a shallowing-upward succession (Fig. 10A). The packstone at the base of the sequence is mainly composed of bioclastic or peloidal packstone with poorly-winnowed fabric (Fig. 7A and B), indicating a moderate-energy intershoal or inter-reef environment. The middle two lithofacies are LF1 and LF2.1 containing Cladosiphonia framestone, bafflestone, and thrombolite (Fig. 5A–D), which represent the major lithofacies forming reef buildups. The uppermost LF3 is mainly peloidal and intraclastic grainstone that was deposited under high-energy littoral environments. The distinct character of LFA1 is that LF1 or LF2 deposits are followed by LF3, indicating a facies transition from reef to shoal under the shallowing water depth and strengthening hydrodynamic conditions, thus forming reef–shoal complex deposits. LFA2 (Fig. 10B) is mainly composed of sediments that were deposited under low-to moderate-energy conditions, including 9

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Fig. 7. Photomicrographs of lithofacies LF4 and LF5. (A) Gastropod-rich packstone containing unbroken spiral shells; Well B197, 1535.5 m. (B) Peloidal packstone with coarse peloidal grainstone clasts and complete ostracods, matrix poorly washed out; Well B182, 1650.3 m. (C) Intraclastic wackestone, clasts showing rip-up shapes; Well 432c, 1575.8 m. (D) Ostracod and gastropod bioclast-rich wackestone; Well 414, 1682.3 m. (E) Scouring structures with coarser fragmented shells at the bottom of scour surfaces; Well 182, 1642.4 m. (F) Undulant, asymmetrical micro-scour surface between bioclastic packstone and wackestone; Well B414, 1660.8 m.

calcimudstone, wackestone, and occasionally interbedded bioclastic packstone in the lower section, and thrombolite or small-columnar stromatolites in the upper section. The major reef contributors to this lithofacies association are non-skeleton thrombolites or small-columnar stromatolites, which represent a moderate-energy environment. These reef-building lithofacies are generally less than 1 m in the 4-to-8-mthick succession, whereas the majority of this succession is thinly laminated and matrix-supported lithofacies representing low-energy deposits. Therefore, this lithofacies association reflects small buildups in lower littoral to upper sublittoral zones (e.g. Cohen and Thouin, 1987). LFA3 comprises three lithofacies, namely wackestone, packstone, and grainstone from bottom to top, showing a coarsening-upward succession (Fig. 10C). The lower lithofacies is matrix-supported, whereas the upper two are grain-supported with abraded and rounded grains, reflecting an increasingly hydrodynamic environment from lowmoderate to moderate-high energy. Accordingly, the vertical shallowing-upward succession could reflect the facies transition from intershoal to shoal in the littoral zones. LFA4 (Fig. 10D) is characterized by intraclastic rudstone or floatstone lithofacies scouring the underlying argillaceous limestone,

intraclastic wackestone, or packstone. In the vertical direction, the underlying lithofacies are matrix-supported lithofacies that represent a low-energy depositional background, whilst the overlying abrupt contact surfaces and intraclastic rudstone or floatstone indicate event flow deposits. Considering the frequent storm activities in the study area (Jiang, 2018; Wang et al., 2015), the whole succession probably represents a high-energy storm event that induced gravity flow deposits in a generally low-energy environment. LFA5 (Fig. 10E) is characterized by mixed lithofacies composed of siliciclastic and carbonate components that reflect the influence of siliciclastic input. This lithofacies association comprises a coarseningupward succession characterized by an increasing amount of siliciclastic components within the rock. The lower mudstone and argillaceous limestone indicate low-energy conditions, whilst the upper arenaceous limestone with wave-ripple beddings reflects moderate-energy environment. This association generally indicates near-landmass siliciclastic-carbonate mixed deposits in littoral zones. 4.3.2. Distribution of lithofacies associations Two cross-sections (NW–SE and W–E orientation) were constructed 10

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Fig. 8. Photographs and photomicrographs of lithofacies LF6. (A) Intraclastic rudstone with non-oriented to slightly oriented gravels; Well B432c, 1600.5 m. (B) Rudstone with poorly-sorted intraclast chips and overlying undulated laminated calcimudstone; Well B197, 1558.98 m. (C) Overlying floatstone scouring former floatstone; Well B182, 1650 m. (D) Intraclastic grain-supported rudstone, with micrite intraclastic gravels supported by tiny micrite fragments; Well B432c, 1600.2 m. (E) Peloid-and-micrite-supported rudstone, gravel consisting of peloidal grainstone or micrite intraclasts, Well B197, 1558.98 m.

to analyze the distribution and spatial variation of LFAs (Fig. 11). They crossed most geomorphological districts, major marginal, and internal synsedimentary faults in the study area. Vertically, HST carbonates are divided into the lower H-1 and the upper H-2 parasequence sets with different lithofacies association stacking patterns, which are generally associated with facies changes. For flat-topped areas (i.e. PFW and SD districts), the H-1 intervals are characterized by the stacked LFA3 cycles, whereas the H-2 intervals are mostly stacked LFA1 cycles, and this transition indicates the vertical facies transition from shoal to reef-shoal (Well B80 and B76 in Fig. 11A; Well B16 in Fig. 11B). For step-fault transitional zones, the stacking patterns could have distinct changes. For example, in the P2 district, the lower LFA4 stacked cycles in H-1 intervals have evolved into LFA3 cycles or LFA1+LFA3 cycles in H-2 intervals (Well B180 in Fig. 11A; Well B182 in Fig. 11B), which may have been caused by littoral shoal or reef-shoal facies progradation to sublittoral zones. In basinward profundal areas, the whole HST interval is dominated by stacked LFA4 cycles (Well B184 in Fig. 11B). Laterally, LFA1 and LFA3 are mostly distributed in two flat-topped areas and were observed as large-scale, continuous and interchangeable facies. LFA2 often occurs in upper parts of transitional zones as small isolated facies. LFA4 is mainly distributed in transitional zones to profundal areas, and LFA5 is more developed near the northern landmass. In the NW–SE-oriented cross-section (Fig. 11A), the SD and PFW districts are dominated by shoal and reef-shoal facies (LFA3 and LFA1), which laterally transform into small bio-buildups (LFA2 in Well B432c) or thinner shoals (Well B180) in upper transitional zones, or transfer to

sublittoral facies that are frequently influenced by storm event deposits (LFA4 in Well B432c, B180). In the W–E-oriented cross-section (Fig. 11B), the facies transition shows an increasing number of storm event deposits (LFA4) from the flat-topped PFW district to its eastward transitional and profundal areas. 5. Discussion 5.1. Depositional model Geomorphologically, the study area was a landmass-attached semiisolated tectonic highland (Fig. 4A), which became drowned with lake transgression and turned to a subaqueous high facilitating carbonate precipitation in the late Es4s period. Siliciclastic-starved subaqueous highs were usually considered as significant and favorable places for carbonate accumulation, which had been summarized as the widely applied “subaqueous high or platform model” in rift basins (Du, 1990; Wang et al., 1993; Zhou and Du, 1986). However, because lacustrine carbonates are quite sensitive to sedimentary environments (Gierlowski-Kordesch, 2010), the above relatively simplistic model is not applicable for detailed facies interpretation in the study area. Lacustrine carbonate deposition is closely associated with factors such as water depth, hydrodynamic conditions, geomorphology, and sedimentary-tectonic setting. For lake margins, geomorphology and hydrodynamic conditions are the two main factors determining carbonate facies models (Platt and Wright, 1991). Based on seismic geomorphology and its relationship with the 11

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Fig. 9. Photographs and photomicrographs of lithofacies LF7 and LF8. (A) Argillaceous limestone with charophyte stem; Well B414, 1663.5 m. (B) Argillaceous limestone with minor amount of quartz grains and shell fragments; Well B414, 1662.7 m. (C) Arenaceous limestone, wave-ripple beddings; Well B16, 1472.4 m belonging to TST. (D) Micrograph of Fig. 9C showing abundant peloid and quartz grains with micrite matrix fill.

Fig. 10. Five lithofacies associations that developed on the carbonate platform. Both LFAs are summaries of LFA characteristics according to cores. The thickness of each succession is as an approximately fifth-order sequence (parasequence). 12

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Fig. 11. Well cross-sections of lacustrine carbonates on the platform showing lithofacies associations, sedimentary environment, and facies distribution. (A) NW–SE trending correlation of Well B80–B180; (B) W–E trending correlation of Well B16–B184.

distribution of different lithofacies and facies, a geomorphology-controlled depositional model was developed including littoral facies in flat-topped areas, sublittoral facies in transitional zones, and profundal facies in deep sags (Fig. 12). Overall, the platform geomorphology was low-gradient above marginal major synsedimentary faults, which could be compared with the high-energy low-gradient “ramp” margins introduced by Platt and Wright (1991). The main difference is that platform internal synsedimentary faults strongly modified geomorphology and controlled different hydrodynamic conditions. In the newly proposed model, the

geomorphology basically controls the general hydrodynamic zones and then determines facies distribution. Additional wind–wave influences were also non-negligible in the study area because frequent storm activities occurred in the late Es4s period (Figs. 8 and 11; Jiang, 2018; Wang et al., 2015), which enhanced gravity flow deposits on the windward sides. 5.1.1. Littoral facies The littoral zone corresponds well with flat-topped areas on the platform. It is characterized by moderate-to high-energy hydrodynamic 13

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Fig. 12. Depositional model of Es4s carbonates in the western Dongying Depression depicting facies distributions in a semi-isolated lacustrine carbonate platform.

conditions, and serves as the main deposition area for reef and shoal deposits. Reefs in this facies are mainly accumulated by tube-like green algae Cladosiphonia, similar to the bioherms built by Cladophorites in the Ries impact crater, southern Germany (Arp, 1995). However, reefforming contributors including Cladosiphonia framestone, bafflestone, thromoblites, and small-columnar to branching stromatolites were too fragile to form rigid and large-scale wave-resistant buildups, and thus they often occurred as small low-relief patchy reefs (Du, 1990; Wang et al., 1993). When the water became shallower (less accommodation space), Cladosiphonia framestone and bafflestone sediments would become less developed because of the relatively strong wave agitation in littoral zones, which could break, erode, and abrade previous reefs to form reworked deposits such as intraclastic and peloidal grainstones. In areas near landmasses, siliciclastic-carbonate mixed deposits became dominated by a large amount of terrigenous clast inputs.

These depositional environments are below the storm wave base and dominated by fine-grained sediments, such as oil shale. However, occasional sediment flows (e.g. gravity flows induced by storm events) could bring some coarse clasts from shallower areas into the deep profundal zone (Kong et al., 2017; Sarg et al., 2013). 5.2. Lacustrine carbonate formation mechanism 5.2.1. Sea transgression Lacustrine carbonates were widely developed throughout the whole basin during the late Es4s period, which coincided exactly with the climate changes from the early cold-arid period to the late relatively warm-damp period (Liang et al., 2018b; Liu, 1998; Ma et al., 2017; Wu et al., 2016). The global climate change curve (Zachos et al., 2001, 2008) reveals a general cooling tendency since the Early Eocene Climatic Optimum event but a minor warming trend occurred during ca. 45 Ma to ca. 42 Ma, which also coincided with the large-scale development of lacustrine carbonates in the Dongying Depression. This global warming may have resulted in a rise in sea level and seawater influx into the near-ocean basin (Fig. 1A, Liang et al., 2018a; Wu et al., 2014), which can explain the widespread marine fossils in the Es4s interval, including Cladosiphonia, Bohaidina granulate, Deflandre, Serpulidae, and Foraminifera (Ge, 1985; Wu et al., 2014). In addition, the warm climate was also favorable for the growth of Cladosiphonia, which facilitated reef development. The warm climate may also have enhanced microbial activities and biologically-induced carbonate precipitation, as indicated by the positive excursion of δ13C values in primary carbonates (Han et al., 2018; Liu, 1998). However, considering that the deposition of carbonate sediments in lacustrine environments is generally associated with a semiarid climate (Gierlowski-Kordesch, 2010; Tanner, 2010), the increased rainfall and siliciclastic influx under the damper climate seems to be unfavorable for the large-scale accumulation of lacustrine carbonates in the Dongying Depression. The increased rainfall in the HST period (high precipitation/evaporation rate indicated by lower δ18O, Liu, 1998) could cause

5.1.2. Sublittoral facies The sublittoral facies are mainly developed in step-fault zones on the platform (Figs. 3, 4 and 12), as transitional areas between flattopped areas and deep-sag areas. The sublittoral zone ranges from fairweather wave base to storm wave base (Flügel, 2010; Sarg et al., 2013), which is generally low-energy hydrodynamic. Under fair-wave conditions, this facies was dominated by intraclastic and bioclastic wackestone, calcimudstone, or argillaceous limestone. In local areas near sublittoral zones, the flat-laminated to wavy stromatolites not associated with grain deposits could accumulate and form small low-relief buildups. Gravity flow deposits are developed under storm-wave conditions, especially in windward sides of step-fault transitional areas. Reworked sediments from in situ or shallower areas could be suspended, transported, and redeposited in this facies to form rudstones or floatstones (Al-Awwad and Pomar, 2015). 5.1.3. Profundal facies The profundal facies are distributed away from marginal major synsedimentary faults, laterally belonging to their adjacent deep sags. 14

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Fig. 13. Geochemical parameters of the Es4s lacustrine carbonates in the Dongying Depression. (A) and (B) PAAS-normalized REE distribution patterns. Different lithofacies are indicated by the different line colors. GRF represents stromatolite carbonate from Eocene lacustrine Green River Formation (Bolhar and Van Kranendonk, 2007). (C) The positive correlation between Y/Ho and La/La*. (D) The linear relationship between Y/Ho and Zr. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

considerable dilution of the aqueous carbonate concentrations, which was unfavorable for carbonate precipitation. Fortunately, in the study area, it was a semi-isolated subaqueous highland far away from main siliciclastic sedimentary systems, so the influences of siliciclastic influx were negligible (Du, 1990; Jiang, 2018; Zhou and Du, 1986). Additional sea transgression brought abundant ions such as Ca2+ and Mg2+ for carbonate formation, which could compensate for the influence of rainfall-induced dilution. Rare earth elements (REE) and post-Archean Australian shale (PAAS)-normalized REE patterns were analyzed to consider the influence that sea transgression brought to the lake basin, such as anomalies in REE concentrations and distribution patterns. Two distinct patterns were observed based on total REE concentrations and the differentiation between light REE (LREE; La, Ce, Pr, Nd, Sm, and Eu) and heavy REE (HREE; Gd, Tb, Dy, Y, Ho, Er, Tm, Yb, and Lu). Firstly, bafflestone, thrombolite, and peloidal grainstone are characterized by relatively lower total REE concentrations and significant LREE depletion

(Fig. 13A), and secondly, stromatolite, wackestone, and argillaceous limestone have higher total REE concentrations and flat patterns without strong LREE or HREE depletions (Fig. 13B). Most REE distribution patterns of bafflestone, thrombolite, and peloidal grainstone were comparable with the typical marine carbonates, which were characterized by positive La, Gd, and Y anomalies and LREE depletion (Bolhar et al., 2004; Bolhar and Van Kranendonk, 2007). These anomalies were obviously different from the lacustrine Green River Formation (GRF) stromatolite sample (Fig. 13A and B). Because distinct changes of REE patterns are generally related to different fluid sources (Nothdurft et al., 2004), the patterns similar to marine carbonates could imply the modification of seawater input during sea transgression. In addition, the Y/Ho ratio values were positively correlated with the La/La* ratio (Fig. 13C), which is also an important feature of seawater-derived carbonates (Bolhar and Van Kranendonk, 2007). However, it is notable that the argillaceous limestone samples had the highest REE concentrations (Fig. 13B), and the 15

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Y/Ho values as marine proxy showed anti-correlation relationship with the continental-derived Zr contents (Fig. 13D). This suggests that siliciclastic contamination was widespread in rift basins that covered the anomalies caused by seawater influx. Therefore, the process of sea transgression should be transitory in the evolution of lake basins. However, considering the widespread marine fossils and other abundant evidence, e.g. ichonofossil Paleodictyon and molecular fossils dinosterane and C13 sterane (Yuan et al., 2007, 2008), glauconite (Wu et al., 2014), and marine-prone inorganic geochemical parameters (Sr/ Ba, B/Ga, bio-Ba, Liang et al., 2018b; Wu et al., 2014), the sea transgression should be substantial and non-negligible for the sedimentary environment of lacustrine carbonates in the Dongying Depression.

windfield and enhanced the prevailing southeast wind (Jiang, 2018; Zhang et al., 2012). The monsoon-influenced climate also caused a higher frequency of storm activities (e.g. typhoon, Jiang, 2018), leading to the common development of storm event flow deposits in windward transitional areas (Fig. 12). 5.3. Implications for reservoir prediction The Es4s lacustrine carbonate in the Dongying Depression is a wellknown carbonate reservoir for oil and gas exploration and development, in which there were four wells that ever produced over one thousand tons of crude oil per day (Du, 1990; Zhou and Du, 1986). Porosities and permeabilities from six types of lithofacies were plotted in the current study (Fig. 14) for application in reservoir analysis and prediction. The best reservoirs are characterized by ultra-high permeabilities even exceeding several darcies (Fig. 14A), which are normally associated with Cladosiphonia-related framestone and bafflestone facies developed in flat-topped areas. However, because strong heterogeneity is present in these lithofacies, from the well-preserved and sparite-cemented lithofacies LF1 to unconsolidated and vuggy ones, their porosities ranged from 8.5% to 47.0% and permeabilities varied from 16.0 md to 4448.5 md. Similar heterogeneity occurred in grainstone lithofacies but the average permeabilities were clearly lower (Fig. 14C), which could be attributed to heterogeneous cementation and dissolution (Zhou and Du, 1986). The boundstone lithofacies that contains inter-skeleton and interparticle pores and high-energy stromatolites (LF2.2B) generally had good porosities and permeabilities, although slightly lower than LF1 and LF3 (Fig. 14B). In the porosity-permeability plots, the packstone lithofacies had low-moderate porosities and low

5.2.2. Wind-wave influences For the study area, major structural trends and geomorphological districts are approximately SW–NW- or W–E-oriented. The SW–NWoriented districts are nearly perpendicular to the prevailing southeast wind and strongly influenced by wind-generated waves from the open lake, hence these districts (PFW in particular) are characterized by welldeveloped high-energy deposits, such as reef-shoal or shoal deposits. The W–E-oriented districts, e.g. SD and S1, cross the prevailing wind direction obliquely, and they also formed under a moderate-to highenergy environment, despite being separated from the open lake. As shown for the NW–SE- and W–E-oriented cross-sections (Fig. 11), the effects of wind and waves strengthened the existing geomorphological hydrodynamic zones and facilitated the large-scale distribution of highenergy sediments on the flat-topped areas (Fig. 11). In addition, “deep-time” paleoclimate studies have shown that an East Asian paleomonsoon climate occurred during Es4s period (Meng et al., 2018; Quan et al., 2012), which strongly influenced the regional

Fig. 14. Porosity versus permeability for major lithofacies types of Es4s lacustrine carbonates in the Dongying Depression. (A) LF1, framestone and bafflestone; skeleton, interparticle porosities. (B) LF2, boundstone; interparticle porosities. (C) LF3, grainstone; interparticle, intraparticle and moldic porosities. (D) LF4, packstone; some interparticle, intraparticle and moldic porosities. (E) LF5, wackestone; a few moldic porosities. (F) LF6, rudstone to floatstone; some interparticle and intraparticle porosities. 16

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permeabilities with wide value ranges (Fig. 14D) and the wackestone lithofacies had the poorest physical properties (Fig. 14E). In comparison, the rudstone and floatstone lithofacies featured moderate–high porosities but relatively low permeabilities (Fig. 14F). The porosity–permeability plots presented in Fig. 14 indicate a close relationship between reservoir physical properties and lithofacies. In addition, geomorphology-associated facies also play an important role in permeability. The littoral facies generally have much better physical properties than sublittoral facies (Fig. 14), and the best reservoirs are distributed in the flat-topped areas. Even for the same lithofacies in different zones, e.g. packstone and wackestone, the lithofacies developed in littoral facies on flat-topped areas of the platform show a distinct higher permeability than those in sublittoral facies on step-fault transitional zones (Fig. 14D and E). The littoral lithofacies (high-geomorphology) might be more easily dissolved to be porous than sublittoral lithofacies during the early diagenesis stage. Nevertheless, sublittoral facies sometimes contain good-quality reservoirs with high porosity and medium permeability (Fig. 14F), especially those distributed in the upper part of transitional zones, such as wavy to smallcolumnar stromatolites (Fig. 5E–G) and shoal-adjacent windward rudstones (Fig. 8B, D–E). As a consequence, with the increasing demand for oil and gas and further reservoir prospection, sublittoral facies on stepfault zones, especially the windward sides, could act as favorable areas for hydrocarbon prospecting.

2017ZX05009-002). We sincerely thank the Geoscience Institute of the Shengli Oilfield, SINOPEC, for providing data access from their inhouse database. We also thank Prof. Marco Brandano and other two reviewers for their constructive comments and suggestions that greatly improved the manuscript. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.marpetgeo.2019.104125. References Al-Awwad, S.F., Pomar, L., 2015. Origin of the rudstone-floatstone beds in the upper Jurassic Arab-D reservoir, Khurais complex, Saudi Arabia. Mar. Pet. Geol. 67, 743–768. Arp, G., 1995. Lacustrine bioherms, spring mounds, and marginal carbonates of the RiesImpact-Crater (Miocene, southern Germany). Facies 33, 35–90. Arp, G., 2006. Sediments of the Ries Crater Lake (Miocene, Southern Germany). Bolhar, R., Kamber, B.S., Moorbath, S., Fedo, C.M., Whitehouse, M.J., 2004. 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6. Conclusions (1) Based on the integrated investigation on seismic geomorphology, lithofacies and lithofacies-association distributions of Es4s lacustrine carbonates in the Dongying Depression, a landmass-attached (semi-isolated) carbonate platform was identified. On the platform, eight lithofacies based on macro- and micro-fabrics (LF1–LF8) were identified, which mainly formed five lithofacies associations (LFA1–LFA5), representing reef-shoal complex deposits, small biobuildups, shoal–intershoal deposits, event flow deposits, and nearland mixed deposits, respectively. (2) A new geomorphology-dominated model was constructed to depict the detailed architecture of the lacustrine carbonate platform. In this model, the three hydrodynamic zones corresponded well to the various geomorphological districts. The littoral, sublittoral, and profundal facies were characterized by moderate-to high-energy, low-to moderate-energy, and low-energy deposits, which were mainly distributed on flat-topped areas, fault-step transitional zones, and deep sags, respectively. Besides the regional geomorphology and its controlled hydrodynamic zones, carbonate formation was facilitated by global-warming-induced sea transgression, which brought abundant marine-sourced bionts and ion-rich seawater. In addition, the paleomonsoon climate enhanced the regional windfield, which also played an important role in the development of lacustrine carbonates on the platform. (3) Porosity and permeability analysis revealed that lacustrine carbonate reservoir properties were strongly associated with the heterogeneity caused by geomorphology and lithofacies. The geomorphology-dominated depositional model for Es4s lacustrine carbonates well reflected the formation and distribution of each lithofacies, which indicates the flat-topped and windward step-fault zones and their dominant deposits could act as potential targets for further hydrocarbon prospection in the platform. Acknowledgments The research presented in this paper was supported by the National Natural Science Foundation of China (No. 41902117), the Open Fund of Key Laboratory of Exploration Technologies for Oil and Gas Resources (Yangtze University), Ministry of Education (Grant No. K2018-07) and the China National Science and Technology Major Project (Grant No. 17

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