Seismic sedimentology of lacustrine delta-fed turbidite systems: Implications for paleoenvironment reconstruction and reservoir prediction

Journal Pre-proof Seismic sedimentology of lacustrine delta-fed turbidite systems: Implications for paleoenvironment reconstruction and reservoir prediction Marco Shaban Lutome, Chengyan Lin, Dong Chunmei, Xianguo Zhang, Dicky Harishidayat PII:

S0264-8172(19)30611-7

DOI:

https://doi.org/10.1016/j.marpetgeo.2019.104159

Reference:

JMPG 104159

To appear in:

Marine and Petroleum Geology

Received Date: 15 October 2019 Revised Date:

1 December 2019

Accepted Date: 3 December 2019

Please cite this article as: Lutome, M.S., Lin, C., Chunmei, D., Zhang, X., Harishidayat, D., Seismic sedimentology of lacustrine delta-fed turbidite systems: Implications for paleoenvironment reconstruction and reservoir prediction, Marine and Petroleum Geology (2020), doi: https://doi.org/10.1016/ j.marpetgeo.2019.104159. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Seismic sedimentology of lacustrine delta-fed turbidite systems: Implications for paleoenvironment reconstruction and reservoir prediction Marco Shaban Lutomea, Chengyan Lina,1, Dong Chunmeia, Xianguo Zhanga, Dicky Harishidayatb a

School of Geosciences, China University of Petroleum (East China), Qingdao 266580, China. Reservoir Geology Key Laboratory of Shandong Province, Qingdao 266580, China.

b

Department of Geoscience and Petroleum, Norwegian University of Science and Technology, Trondheim 7031, Norway.

Abstract The deltaic-turbidite systems are common features in continental lacustrine rift basins. In the Bohai Bay rift Basin, Eastern China, the turbidite sandstones are the critical targets for hydrocarbon production and development. However, their intricate sedimentary patterns and the small size of the reservoirs introduces more challenge for their understanding and prediction. Through seismic sedimentology workflow, 90°-phase adjustment, strata slicing, spectral decomposition, RGB color blending technique, and geobody extraction, 3D seismic data and wireline log information data are integrated to understand the historical evolution of delta-turbidite systems and predicting the spatial and temporal distribution of sandbodies of the Es3m sub-member in the Shahejie Formation, dongying depression. The study identifies two system tracts in the study area, transgressive and highstand system tracts, from seismic and wireline logs interpretation. We identified six fourth-order sequences within the highstand system tracts, each of which corresponds to a different period of deltas progradation. Stratal slicing mapping on multiple deposition surfaces reveals the highresolution historical evolution of the delta-turbidite systems and the distribution of their associated sandbodies. Interpretation of slices shows six stages of deltas development, which led to widespread delta-fed turbidite reservoirs. Three multi-delta-turbidite systems: the northern fan-turbidite systems and the southeast-turbidite systems are documented, which 1

Corresponding author: School of Geosciences, China University of Petroleum (Qingdao), 266580, PR China. Email: [email protected] 1

evolved throughout all stages of delta development. Rapid progradation of multi-deltas systems played an essential role in the evolution and development of thin-bedded reservoirs in the HST. Faults and local topographic lows controlled the spatial distribution of sandbodies in the area. The individual sandbodies range from 4.48 km2-16.49 km2 in size, and they vertically superimposed from bottom to top. The classification scheme presented in this work shows a successful application in terms of paleoenvironmental reconstruction and reservoir prediction of the delta-fed turbidite systems, which can be applied to similar systems worldwide. Keywords: Lacustrine delta-turbidite systems; Dongying depression; seismic sedimentology; Stratal slicing; Spectral decomposition; Geobody extraction;

1

Introduction

Sediment flows are gravity flow-controlled processes that transport and distribute sediments downslope and into deeper areas (Middleton and Hampton, 1973; Lowe, 1982; Fisher, 1983). The emphasize of turbidity flow deposits in recent years have been recognized by their importance in natural hazard investigation, oil and gas exploration, and climate change forecast (Embley, 1976; Heller and Dickinson, 1985; Weimer and Link, 1991; Shanmugam et al., 1994; Hampton et al., 1996; Mutti et al., 2003). The turbidity currents and their associated deposits perhaps represent the vital step in sedimentological studies in both marine and lacustrine rift basins (Zavala and Arcuri, 2016). The fault break zones in lacustrine rift basins have been related to continental shelf-slope systems in marine settings (Mutti et al., 2003; Covault et al., 2009; Xiugang et al., 2014; Feng et al., 2016). Compared to marine basins, the rift basins are characterized by complex tectonic history, which influences the sedimentary pattern of sedimentary deposits (Zhang and Scholz, 2015). Deltas systems are commonly developed in rift basins as the results of the delta-fed turbidites are prevalent during the period of deltas progradation. The sedimentary architecture of the turbidites in these basins is 2

controlled mainly by accommodation space available on the slope and in the depocenters (Kneller, 2003). Although deep-water turbidities are commonly developed in both marine and continental rift settings, most of the research has focused on marine basins, and there has been less documentation on the lacustrine rift basins (Zhang, 2004). In the Dongying depression (Fig.1B), the delta-turbidite systems are well-developed in the third member (Es3m) of the Shahejie Formation. Both systems form the basis for turbidite reservoirs development and provide the structural and stratigraphic potential for oil and gas exploration. The development and distribution of turbidite sandbodies in the depression is controlled mainly by faulting activity, sedimentation rate, and less lake-base level changes (Feng et al., 2016). The more recent studies have focused on turbidite reservoir geometry, architecture and sedimentary characteristics (Liu et al., 2016; Liu et al., 2017; Munawar et al., 2018) and sequence-stratigraphic analysis from core descriptions and well log and seismic data interpretation (Zhang, 2004). However, the presence of high fourth-order sequences poses problems in understanding and predicting these delta-turbidite fed sandbodies. The turbidite reservoirs within the fourth-order sequences are usually thinner and have limited scales that introduce problems for their prediction. This study uses seismic sedimentology to predict and characterize these thin-beds of sandstone reservoirs, which are delivered by multi-delta-turbidite depositional systems (Xian et al., 2018). The details of the seismic sedimentology method are given by (Zeng and Hentz, 2004; Zeng, 2007). It offers several disciplines, including seismic geomorphology (Posamentier and Kolla, 2003; Posamentier, 2005), lithology, depositional architecture, and history. The 90° phase readjustment (Zeng and Backus, 2005a; Zeng and Backus, 2005b) and stratal slicing (Zeng et al., 1998) are the fundamental techniques used for seismic lithologic and seismic geomorphologic studies respectively. This study aims to use seismic sedimentology workflow (1) to improve the understanding of the evolution of depositional systems (2) to predict the distribution of thin

3

beds of sandbodies in the (Es3m) Formation interval in the Dongying Depression, Bohai Bay Basin.

P’

B

N

A

Study Area

Base map

0

5 km

D

P

Fig.1: Geological map of the Bohai Bay Basin and Geological cross-section of the Dongying depression (PP’), eastern China. The basin comprises Jiyang (I), Huanghua (II) Bozhong (III) Zhezhong (IV) Liaohe (V) Dongpu subbasin (VI) subbasins. The geological cross-section represents the structural zones within the dongying depression (as modified from (Munawar et al., 2018). The base map (D) represents the outline of the 3D seismic survey area.

2

Geological settings

The Bohai Bay Basin is a Mesozoic-Cenozoic rift basin that is viewed one of the essential hydrocarbon-producing regions in Eastern China covering a total area of about 200,000 km2 (Allen et al., 1997; Guo et al., 2012), and it is composed of six subbasins (Changming et al., 1984). The Jiyang subbasin constitutes one of the significant six subbasins of the Bohai Bay Basin. The Dongying depression (Fig.1B) is part and located in the southeastern of the Jiyang subbasin. It is a typical half-graben with a faulted northern and southern borders (Cao et al., 4

2014) covering the total area of approximately 5700 km2. It is composed of four subdepressions, namely Lijin, Mifeng, Boxing, and Niuzhuang. The Chenjiazhuang uplifts bounds it to the north, Luxi uplifts to the south, Qingtuozi-Guangrao to the east, and Qingcheng Linjia-Binxian to the west (Xian et al., 2018). From the north to the south, it segregated into five structural zones Fig.1C: the northern steep slope zone, northern sag (Lijin sag) zone, central diapiric anticline zone, southern sag (Niuzhang sag) zone, and the gentle southern zone. These structural zones are the main depocenters for sediment accumulation (Guo et al., 2010). The series of normal faults extending into the basement divided the depression and controlled the distribution of sedimentary systems in the basin (Li, 2004; Guo et al., 2012). Tectonically, Dongying depression experienced two significant phases of tectonic evolution. The syn-rifting stage which occurred in the Paleogene period from 65.0-24.6 Ma. Furthermore, the initial rifting phase, rifting expansion phase, deep subsidence, and contraction phase forms the subdivisions of the syn-rifting phase Fig.2. The thermal subsidence phase occurred after the deposition of the Dongying Formation in the Neogene from 24.6 Ma to the Quaternary period (Xian et al., 2018). Stratigraphically, the deposition in the dongying depression is dominated by a thick succession Cenozoic sediments, which are mainly dominated by non-marine sandstones and mudrocks with minor carbonates and evaporates (Zhang, 2004). It comprises five Formations: The Paleogene Kongdian (Ek) Formation, the Shahejie Formation (Es), and the Dongying (Ed) Formation; the Neogene Formations include the Guantao (Ng) Formation and the Minghuazhen (Nm) Formation, and Quaternary Pingyuan Formation (Fig.2). The Shahejie Formation (Es) is divided into four members namely Es4, Es3, Es2, and Es1, from bottom to top. The Es3 member that is the focus of the Formation was deposited during basin development when deep lacustrine basins formed. It is dominated mainly by lacustrine oil

5

shales, dark-grey mudstone, calcareous mudstone, and thin intercalated sandstones (Guo et al., 2012; Liu et al., 2014). This member formed the significant source rocks within the depression. The Es3 member is subdivided into sub-members namely lower (Es3l), middle (Es3m), which is the study interval, and upper (Es3u) (Lampe et al., 2012). Streams emanating from topographic highs carried clastic sediments formed large scale deltas and frontal deltaturbidites systems in deep parts of the Dongying depression. On the northern slope in the shallow and lakeshore environments, local alluvial fans and fan deltas developed. The main faults defining the grabens and half-grabens and the prograding deltaic clinoforms near the center of asymmetrical basins generally played a significant role in the development of lacustrine delta systems in the depression (Chen et al., 2009).

6

Fig.2: A generalized chronostratigraphic sequences and structural evolution of the Dongying depression (Modified after (Munawar et al., 2018; Xian et al., 2018)

3 3.1

Data and Methods Dataset

The study used 3D seismic reflection data covering an area of 300 km2 with bin spacing of 12.5 m x 12.5 m. The seismic data is processed to zero phase and has normal polarity. The peak (black reflectors) has shown increasing acoustic impedance, and the trough (red) has a decreasing acoustic impedance. The seismic data have the dominant frequency of approximately 31Hz sampled at 4ms with frequency ranging from 0 to 70Hz, which yields a vertical resolution of about 12.5m. Fifty-six wells with the wireline logs integrated with the seismic data were used to the established stratigraphic framework of the area, in addition to sequence boundaries and system tracts. 7

3.2 3.2.1

Methodology Seismic and wellbore data interpretation

The wireline logs and seismic data were integrated to identify and map the high fourth-order sequences, which are the results of multi-delta progradation. Interpretation of wireline log responses and seismic facies analysis was adopted to establish the stratigraphic framework and to recognize the deltaic clinoform systems associated with fourth-order sequences in the study interval. 3.2.2

Seismic sedimentology

A 90°-phase adjustment for the lithologic wireline logs-seismic correlation was applied to the 3D seismic data. This correlation used to establish the relationship between thin beds (sandstones) and seismic events for accurate mapping and interpretation. The relative time geological model (RTG), simply geomodel, was created using a global optimization interpretation approach (Pauget et al., 2009). All the fourth-order sequences within the highstand system tracts were interpreted to provide a high-quality and highresolution geologically consistent model. The model offered full dimensionality of the seismic data for accurate mapping. Faults and external seismic horizons T4 and T6, which are geological significance, were introduced in the model-grid during interpretation as the geological constraints to refine and reflect the stratigraphic geometry of the formation interval. These seismic reflection horizons (T4, T6) were picked manually in Schlumberger Petrel Software and imported into PaleoScanTM. The stratal slicing mapping tool was applied after model computation to map the depositional systems and their associated delta-turbidite fed sandbodies at comparatively high-resolution. One-hundred and ninety-five (195) stratal slices were extracted from the 3D model and 3D seismic data as the primary inputs. The (T4, T6) seismic horizons were used as bounding

8

reference surfaces for better mapping of geomorphological features of the depositional systems within the delta-turbidite systems. Spectral decomposition and RGB color blending techniques were carried out to improve the understanding of thinly turbidite sandbodies. Three seismic volumes from the spectrogram, which represent the signal of the geological targets, were picked and created. These three volumes corresponding to three different remarkable frequencies centered at 10Hz, 15Hz, and 40Hz were mapped on horizon stacks in the RGB color blending window using short time Fourier transform centered at 27ms window size. These three volumes were assigned to blue, green, and red respectively on RGB color blending viewer. The amplitude anomalies of the depositional features identified on horizon stacks during interpretation were used to extract geobodies related to sandbodies. The automatic geobody extraction technique was adopted on the horizon stack to improve the understanding of the thin turbidite sandbodies distribution and their dimension in space. Fig.3 represents the summary of the workflow used in the study.

9

Fig.3. The summary of the flow methodology for the study and the type of data used in the interpretation.

4

Results

4.1 4.1.1

Seismic stratigraphy Seismic reflection termination pattern

Seismic stratigraphy (Mitchum Jr et al., 1977) is the critical aspect in subdividing the stratigraphic interval into depositional units. It serves as the foundation to establish a geological framework of the area and to understand the depositional environments from seismic facies analysis and reflection characters. Fig.4 shows the seismic section with T4 and T6 seismic reflections being the top and bottom sequence boundaries of the Es3m Formation interval, respectively. These surfaces display continuous, stable, and high amplitude across the survey area. The base surface T6 is identified based on local truncation on strata below it (yellow dashed line) with no apparent onlap of downlap above the layers. The top surface (T4) locally terminates the strata below it with no evident termination above layers. Seismic reflection terminations are the characteristics of the fourth-order sequences (green lines) against their high-order surfaces. Each of the high-order surfaces comprises parasequence sets, which are characterized by sigmoid clinoform geometry, low to moderate reflections amplitude, medium to low frequency, chaotic, less continuous reflections. The parasequence sets terminate downlap basinward (down black arrow) and onlapping towards the basin edges (up black arrows). A A’

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4.1.2

Sequence stratigraphy

Sequence stratigraphic interpretation of wireline logs identified two system tracts in the study interval. The (Catuneanu, 2006) interpretation scheme was adopted to understand the regional stratigraphic framework of the area. The transgressive system tract (Fig.5), marked by the double red arrow, is characterized by upward fining-up gamma-ray log trends close toward its top. The thickness of the transgressive system tract is limited but widely developed across the basin, and it depicts high amplitude and continuous seismic reflection. The dominant lithologies are mudstone, siltstone, and other fine-grained rocks. Repeated coarsening-upward gamma-log response trends characterize the highstand system tract (pink double arrow). It is composed of six fourth-order sequences produced as the result of delta progradation at different evolution periods. Each of these corresponds to delta progradation periods and inhibits progradation trends. These deltaic clinoform strata onlap up-dip and termination B’against their fourth-order surfaces. The dominants lithologies include mudstones, downdip B

siltstones, fine and coarse-grained sandstones deposited by gravity flows, or through slumping from the delta-fronts.

HST

TST 11

Amplitud

-

+

Bottom boundary

Top boundary

4.2 4.2.1

Seismic lithology Phase adjustment of seismic data

The 90°-phase shift of seismic data (Zeng and Backus, 2005a; Zeng and Backus, 2005b) has provided significant benefits for thin-bed analysis by enhancing the correlation between lithologic logs and seismic amplitude traces. The majority of the seismic reflection events of Chana's rift basins are produced by thin beds of sandstones (Dongna et al., 2014). However, they lack direct relationships with sandstones, and thus the zero phased seismic data become less applicable for the interpretation of thinly bedded sandstones. The 90°-phased seismic data takes advantage by shifting the maximum amplitude response of seismic data to the midpoint of thin beds. In the studied interval, the sandstones as presented low gamma-ray logs on the vertical section shows poor correlation with seismic data on zero phase seismic section Fig.6a. In this section, the top-half and bottom-half part of the sandstone bodies correspond to seismic peak amplitude (black) and seismic trough amplitude (red), respectively. However, on the conversion of seismic data to 90°-phase, there is a relative improved relationship between seismic reflection events and thin-bed of sandstone Fig.6b. The sandstones almost tie to the seismic peak (black), whereas the mudstones are nearly tied 12

to the seismic trough (red). Generally, the 90° phase improves the seismic-lithologic correlation. In this study, the seismic reflection events can be treated as lithologic units for thin-beds interpretation.

C

TWT (s)

C’ a

TWT

b

13

4.3 4.3.1

Seismic geomorphology Stratal slicing

Seismic geomorphology employs contemporary sedimentology and geomorphology of significant depositional units in predicting depositional facies (Hongliu et al., 2012). To map the depositional facies and to understand the reservoir distribution, it uses the stratal slicing technique, which is generated between two seismic reference surfaces of geological significance. Construction or building of precise seismic-stratigraphic framework between two relative geologic-seismic time surfaces in the defined interval is the most critical task before extracting slices at multiple depositional surfaces. The choice of the particular slicing technique mainly depends on the stratigraphic and structural situation of the studied area. If the sheetlike and flat-lying formation dominates the area, the time-slicing method works perfectly. If the area is composed of sheetlike but not flat-lying formation, the horizon slicing method is the second option; otherwise, the stratal or proportional slicing method is the preferred choice. The stratal slicing takes into account the thickness variation of the Formation (Zeng, 2007), which provides an advantage over the first two methods, and therefore is adopted for this study. Fig.7 illustrates the three seismic slicing methods used to extract attributes on different depositional seismic surfaces.

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Fig. 7. Shows a systematic illustration of the three different seismic slicing methods for different reference seismic surfaces.

4.3.2

Building of Geological Time Model (Model)

The construction of a high-quality relative time geologic model is the crucial procedure that facilitates stratal slicing. The stratal slicing allows a smooth geomorphologic mapping of the depositional systems and depositional facies. The depositional features of the high-order sequences are hard to resolve in the vertical seismic section. Still, they are easily resolved when the seismic attributes are displayed on the relative geologic time surfaces (Zeng et al., 2001). The study interval is marked at its top and bottom by T4 and T6 seismic reflections respectively. These reflections are characterized by stable seismic amplitude, regionally correlative, and easily picked for the entire area. They are manually interpreted and used as the top and bottom model boundaries. The relative time geologic model was interpreted and created using a global optimization and interpolation approach; as the results, all the fourthorder sequences were accurately interpreted on peaks and trough polarity to produce the highresolution and more accurate geological model. Fig.8 shows the geomodel in 2D and the original seismic as the background. The model contains many horizons, each corresponding to different geological ages, and therefore establishes a seismic-time stratigraphic framework of geological consistency. It allows a range of applications, including stratal slicing for geological mapping of depositional

15

features. A large number of stratal slices (195) derived from the geomodel between T4 and T6, and the results are shown in Fig.9 and Fig.10. The stratal slicing was carried using PaleoScanTM with a route mean square (RMS) display seismic attribute.

Fig. 8. Shows the geomodel in 2D (colored mid interval) and the original seismic section (black gray). All fourth-order sequences are interpreted to produce highresolution model of geological consistency for stratal slicing and other applications. and are used in model building. Note: the third-order sequence surfaces are used to constrain the model-grid and the geomodel.

4.3.3

Stratal slices interpretation

The exceptional key feature of stratal slices is the ability to display a full range of depositional systems at relatively high-resolution. Six (6) stratal slices (Fig.9 and Fig.10) have been chosen to demonstrate the evolutionary history of the delta-fed turbidite systems and for facies interpretation. Each of the selected slices (Fig.14) corresponds to an independent period of delta progradation within the highstand system tract. Stratal slices (SS1-SS6) (Fig.9 and Fig.10) are characterized by strong to weak positive amplitude (red) dispersal patterns. Interpretation of seismic lithology and gamma-ray response reveals that the strong to moderate amplitudes correspond to delta sediments. In contrast, the medium to weak amplitudes relates to turbidite facies or deposits. The black amplitude displayed on the slices corresponds to shale, mudstone, or other fine-grained sediments; these non-reservoir facies are exhibiting high gamma-ray response in most wells.

16

Based on geomorphologic amplitude distribution trends, the slices reveal three multi-delta depositional systems during the deposition of the Es3

m

Formation. Each system and its

associated turbidite-fed facies evolved at different phases of the deltas progradation period. The three multi-delta depositional systems are the northern fan-turbidite, the northeastturbidite, and the southeast delta-turbidite systems. Generally, the interpretation of RMS amplitudes displayed on the stratal slices demonstrates the full understanding of the historical evolution and sandbodies distribution in the Es3m. However, no channels are observed in the studied area.

a

a

b

b

17

18

a

a

a

b

b

b

Fig. 10. Seismic stratal slices illustrating the evolution of delta-turbidite depositional systems and their associated turbidite-fed sandbodies from stage four, stage five and stage six of the deltas progradation periods. The red amplitude matches with sandstone and the black amplitude matches with shale or mudstones. The position of the slices on vertical seismic section are shown in Fig.14

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4.4

Spectral Decomposition and RGB color Blending

Frequency components contained in the seismic data provide useful geological features that respond to different frequencies. Geological features such as channels and sandbodies have different spectral responses in the seismic data frequency spectrum (Koson et al., 2014). Because of the turning effects, a specific feature can be easily visible at a particular frequency range. Also, the geomorphological information of a specific geological feature can be better visualized at different frequencies. Spectral decomposition is effective in resolving thinbedded sandbodies. Fig.11 shows the result of three blended volumes of markable frequencies 10Hz, 25Hz, and 40Hz. The results reveal the variation of frequency response on different geological features. The thin-bedded sandbodies represented by brownish-red color are better highlighted and are more visible on blended horizon stack. Some sandbodies which were not resolved on stratal slices stand out clearly on the mixed image. For example, the zone represented by the red dashed circle was not resolved on stratal slice number three or stage three of Fig.9, but it stands out clearly along with its extent on the blended image. N

Fig.11: RGB blend composed of spectral decomposition frequency response volumes showing turbidite depositional facies. The thin-bedded sandbodies, which was not clearly delineated on stratal slice in stage three Fig.9 stand out clearly as represented by a red polygon circle on blended image.

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4.5

Geobodies Extraction

Geobodies are three-dimensional objects representing geological features such as channels that may influence reservoir characterization and prediction. They are extracted from seismic data with the aid of various seismic volume attributes performed on the 3D seismic volume or the horizon stack (Chaves et al., 2011). These attributes are useful in identifying and isolating the geological targets such as structural or depositional features to be extracted. The geobody extraction is an essential procedure in seismic interpretation to enhance reservoir understanding and facies definition. Fig.12 shows the three-dimension geobodies extracted using the automatic extraction process in PaleoscanTM. The RMS amplitude mapped and isolated the delta-fed depositional facies (sandstones) on the horizon stack by employing color bar mode. These depositional bodies display high amplitude anomalies on the horizon stack and extracted as independent geological objects within the studied zone. The individual geobodies range from 4.48 km2 to 16.48 km2 in area, and they correspond to red amplitude on stratal slices (Fig.9 and Fig.10). Stratigraphically, they display a vertical offset stacking configuration or superimposed from bottom to top of the Formation interval.

21

a

b

Fig.12: Automatic extraction of geobodies representing the individual three dimentional turbidite-sand facies. Both (a) and (b) depicting the geometry, lateral, and temporal distribution of geobodies during deltas progradation periods.

5 5.1

Discussion Lacustrine delta-turbidities systems

The selected stratal slices have revealed the extent of depositional systems and their historical evolution in the study area. The evolution of these systems is being linked and correspond to 22

different phases of delta development during the deposition of the Es3m sub-member in the highstand system tract. Six stages of deltas development and three multi-delta and their associated turbidite systems are recognized on the amplitude stratal slices Fig.9 and Fig.10. These multi-delta depositional systems experienced a significant evolution to produce deep lacustrine deposits. The deltas systems include the northern fan systems, the northeast systems, and the southeast, or dongying deltas systems are the critical source of sediments to the deep areas. In each development stage, at least one delta systems were active to supply sufficient sediments to the basin center. For stance, the northeast delta system was engaged at stage one and two but abandoned at stage three during which the southeast delta systems emerged sufficiently to produce sufficient delta-fed turbidites. The stratal slices suggest that rapid progradation of deltas occurred at stage three Fig.9, during which widespread delta-fed turbidites dominated the basin depression. At this stage, a sufficient quantity of sediments was deposited in the deeper parts of the basin to form turbidite rich deposits. The last stage (Fig.10) perhaps represents the period of slow delta progradation. During this stage, delta-fed turbidites are distributed close to deltas with a limited distribution in deeper areas. The variations in amplitude plain view on the slices can be related to the amount of clastic terrigenous sediments delivered to the basins. Rapid multi-deltas progradation is one of the critical factors for the development and distribution of sandstone reservoirs in the dongying depression. In lacustrine basins, base-level change and slope gradient are the essential aspects for the development and occurrence of gravity-derived flow deposits; these factors promote sediment instability at the delta fronts to form widespread deep-water deposits (Hampton, 1972; Kneller, 2003). Sufficient sediment supply and base-level changes can play a vital role in the development and distribution of delta-fed lobes in the depression. The turbidite sandbodies

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presented by red amplitude on stratal slices are well- developed and widely distributed, which might have been influenced by thereof parameters. 5.1.1

Sandstones distribution

Faults play significant roles in the distribution of sandstone reservoirs in most rift basins. The amplitude dispersal patterns on stratal slices reveal the spatial distribution of sandbodies. Turbidite sandstone bodies are well-developed for each stage of deltas evolution. Wireline correlation (Fig.13) reveals that the sand bodies are vertically stacked with the small vertical offset basinward but enclosed by mudstones. The turbidite sand-rich bodies depict low gamma values within the mudstone rich-deposits. In the dongying depression, the Es3m constitute the main source rocks (mudstones). However, during an intensive rifting phase, a large number of terrigenous materials slumped to form turbidity deposits with sandstone being lenticular bodies encased within the source rocks(Zhang, 2004). Geobodies extraction provides an additional reservoir understanding of the three-dimensional of sandbodies Fig.12b. These geobodies range from 4.48km2 to 12.149km2 in the areas. They have lenticular lobe shapes on logs that thicken in the middle and thin at both ends. A considerable accumulation of sandbodies occurs on local depressions and along the axis of the fault troughs. The amplitude distribution pattern reveals that the sandbodies are distributed along the middle slope of the faults and in local deep depression areas found in several parts of the basin Fig.15a and b

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E’

E

Fig.13: Wireline-log stratigraphic correlation EE’ (see location in Fig.1B) demonstrates the turbidite sandbodies in the studied interval for different deltas progradation periods. The turbidite sandstone reservoirs are stratigraphically superimposed but vertically detached by the thick mudstone or fine-grained rocks.

D

D

T

SS6 SS SS SS SS SS

T --

Amplitud

+

Fig.14: 2D seismic section DD’ (see location in Fig.1D) shows the position of stratal slices (black dashed lines) for all six stages shown in Fig.9 and 10.

5.1.2

Factors influencing sandbodies distribution

The syndepositional faults and local depressions developed during rifting periods in rift basins form the space for sediment accumulation (Feng et al., 2013; Feng et al., 2016). The faults form local gradients which promote sediment instability similar to shelf edge in marine settings (Covault et al., 2009). Fig.15 provides an understanding of the influence of faults for 25

the development and distribution of delta-fed turbidites. The faults appear to control the distribution and accumulation of sandbodies in the basin — the sandstones accumulated on the lower parts of these faults. The presence of local depression provided accommodation space for sedimentary deposits. Stratal slices for stages one and two are used to illustrate the importance of faults and local depression in the basin in distributing turbidite sandbodies in the basin. The sandstone occurs along the axis of the normal faults at their base slope and in depressions, where a significant accumulation of sands deposited.

Fig.15: Illustrates the role of faults and local topographic lows in controlling the

distribution of turbidities sand bodies in the in the depression. The sandstone reservoirs are mainly distributed along the troughs of the normal faults (a) and in local topographic lows (b). Note the Figures represent the stage one and three as shown in Fig.9.

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6

Conclusions

Like other deep-turbidites developed in passive margin basins, the turbidites formed in rift basins as in the dongying depression are recently becoming potential for hydrocarbon research and development. (1) The amplitude distribution pattern of the selected stratal slices has revealed three delta-fed turbidite systems during the deposition of the Es3m Formation. These systems, including the northern fan-turbidite systems, the northeast-turbidite systems, and the southeast-turbidite or the dongying delta systems, experienced multiple development stages. (2) The stratal slices have identified six different deltas progradation periods within the highstand system tract. The evolution of the delta systems throughout the six development phases lead to the development and distribution of widespread turbidites and their associated sandbodies. (3) Stratigraphically, the sandbodies have a vertical offset stacking pattern towards the basin but separated by mudstones vertically, and they range from 4.48 km2 to 16.48 km2 in areas. (4) The sandstone reservoirs are better development for each development phase, and the faults and local depressions control their distribution and accumulation.

Acknowledgments The work has been financially supported by the National Science and Technology Major Project of China (Grant number 2016ZX05027-004-002 & 2016ZX05031001-001-003) and the National Natural Science Foundation of China (Grant numbers 41672129 & 41772139).

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Highlights •

The objective is the evolution of depositional systems and predicting the spatial and temporal distribution of delta-fed sand bodies in the high-order sequences.

•

Sequence boundaries and the associated parasequence sets were recognized from wireline logs and seismic data

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Delta-turbidite systems and their associated delta-fed turbidite were delineated using the stratal slicing method.

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Sandstone reservoirs were predicted from successive stratal slices interpretation.

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Rapid delta progradation and sediment supply play an essential role in distributing the evolution of turbidite sandstone reservoirs.

Authors Contributions Marco Shaban Lutome: conceptualization, methodology, software application, writing-original drafting. Chengyan Lin: data and funding acquisition and project management/ supervision. Dong Chunmei: Supervision and well log interpreter. Xianguo Zhang: Data and results interpretation. Dicky Harishidayat: writing-review and editing

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: