Jiang Hui. CHINA PETROLEUM EXPLORATION Volume 23, Issue 5, September Sinopec International Petroleum Exploration and Production Corporation

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1 CHINA PETROLEUM EXPLORATION Volume 23, Issue 5, September 2018 The depositional characteristics and distribution law of shallow-water delta during late rifting stage: a case study on the upper Oilgocene in the Sunda Basin, Indonesia Jiang Hui Sinopec International Petroleum Exploration and Production Corporation Abstract: Plenty of geological and geophysical data show that shallow-water deltas were developed during the late Oilgocene rifting stage in the Sunda Basin, Indonesia, of which deltaic plains were dominant, but deltaic fronts and prodeltas were less developed. Controlled by shallow basin, gentle slope and rapid current, primary hydrodynamics influencing the shallow-water deltas are surface divergent runoff and vein traction flow dominated by seasonal meandering rivers and braided rivers. Distributary channel and natural levee are main depositional microfacies with abundant sandstone. The former acts as an unloading terminal and a passing channel. The latter gradually accumulates and replaces the inter-river sand bar while transporting. Marsh and coal seam strata are more developed, but they are difficult to shape duringgravity deposition. The climate is an important factor affecting the shape of the shallow water delta sand bodies in the late rifting stage. Under the action of thermal diffusion and condensation, the periodic exposure alternating with the rising of the water in the lake basin caused frequent bifurcation, diversion and superposition of the river channels, and developed typical patterns of delta sand bodies, such as leave, teeth, branch, strip. Key words: Sunda Basin, late rifting stage, shallow-water delta, depositional characteristics, distribution law Generally speaking, shallow delta refers to deltas formed under the conditions of shallow water body, gentle terrain, relatively stable structure, relatively slow subsidence [1 5]. Fisk first proposed the concept of shallow delta. Donaldon et al. found that water depth is an important controlling factor, and further summarized the concept of shallow delta. The idea that water depth is the primary factor controlling the development of shallow deltas has been widely accepted by scholars at home and abroad. However, the definition of shallow water does not yet have a universal understanding. Additionally, topography and slope of lacustrine bottom, fluctuation of sea level, the amount of injected sediment, and the climate are also important factors affecting the development and distribution of shallow deltas [6 9]. Mei Zhichao, Lou Zhanghua, Zou Caineng, Zhao Hanqin, Zhu Weilin, et al. conducted further discussions from the aspects of facies sequence and sedimentary framework, controls of physical medium on water environment, sandbody distribution law, classification of sedimentary models and submodels, and the relationship between reservoirs and oil and gas exploration in shallow deltas [10 14]. A shallow delta originally belongs to a fluvial-dominated delta, in particular, a large shallow delta body, which is generally developed on the neritic facies, shallow lacustrine basins, or shallow areas in lacustrine basin with flat terrain. This is also one of the major reasons that this type of delta is mainly concentrated in eperic seas and depression basins with slow and stable subsidence [15 16]. Rift basins or graben basins are widely distributed at home and abroad, with various tectonic sedimentary types and a close relationship with oil and gas accumulation. However, there are few intensive studies on sedimentary distribution in the formation conditions and evolution of shallow deltas during late rifting stage of rift basins or depression stage of graben basins. Most of the rift basins or graben basins have difficulty meeting the conditions of shallow deltas in terms of water depth and tectonic movement intensity in their primary evolution stages, but during late rift periods of rift basins or during the depression periods of graben basins, the accommodation space gradually reduces due to continuously weakening of tectonic movement and enhanced sediment filling and denudation. Especially in the gentle slope zones of the basins, a large scale of wide and gentle shallow water regions can be formed in gentle slope zones of the basins and the shallow water deltas with a certain scale can be developed under the condition of sufficient provenance supply. This paper attempted to deconstruct the sedimentary framework and its distribution law of shallow delta during late rifting stage of rift basins by using four coring wells (with core length of 118 m), well logging and mud logging data of 46 wells, and 3D seismic data of km 2 combined with previous research Received date: 11 Aug. 2017; Revised date: 29 Jun Corresponding author. hjiang.sipc@sinopec.com Foundation item: National Science and Technology Major Project Key Technologies for Oil and Gas Exploration in Deep sea water regions (2011ZX ). Copyright 2018, Petroleum Industry Press, PetroChina. All rights reserved.

2 2 CHINA PETROLEUM EXPLORATION Vol. 23, No. 5, 2018 achievements [17 20]. 1. Microfacies and facies marker of shallow deltas The Sunda Basin, which is located in the northern part of the strait between Sumatra Island and Java Island in Indonesia, strikes NS and covers an area of about 7000 km 2. During the Paleogene Eocene, the basin was developed into a rift basin with graben structure, which was composed of multiple sags (Fig. 1). A set of continental clastic rocks was deposited with a maximum sediment thickness of 5000 m. The basin can be subdivided into three evolutionary stages during the Oligocene rifting period [21] : the initial rifting period, which is equivalent to the depositional period of the Banuwati Formation (about Ma); the rifting expansion period, which is equivalent to the depositional period of lower and middle Zelda member in the Tarlang Formation (about Ma); and the late rifting period, which is equivalent to the depositional period of the upper Zelda member in the Tarlang Formation (about Ma). The tectonic rifting process of the basin had a short duration (9.9 Ma), and the lacustrine basin was smaller and narrow and shallow. During the late rifting stage, two shallow deltas characterized by shallow water were developed from the western slope and the southern uplift. but the foreset sandbody and bottomset (delta front and prodelta) were not completely developed. Among them, microfacies such as the distributary channel, interdistributary bay, natural levee, and back-bank swamp can be identified in delta plain subfacies. On seismic sections, they are characterized by a series of progradational configuration with wedge-shaped morphology and imbricated internal reflection structure. Medium amplitude, medium-weak continuity, and medium-low frequency reflect the progradation of strong water flow in the gentle slope area or shallow area. Due to the gentle slope and shallow waters, gravity flow deposits such as the turbidity fan on the front end of delta and turbidite fan on the far shore were not developed. This is the comprehensive reflection of flat lacustrine topography, weak hydrodynamics in the estuary area, and the stripped and dendritic inflow to the lacustrine of seasonal sedimentary sandbody by progressive extension [22] Distributary channel The distributary channel is the main body of this shallow delta. The provenance direction is northwest. The lithologies are dominated by medium sandstone, coarse sandstone and gravel sandstone, showing tabular cross bedding and trough cross bedding (Fig. 2a). The bottom contacts the underlying strata with scouring erosion and contains residual sediments such as plant roots, reflecting the depositional characteristics of seasonal strong hydrodynamics. The particle size probability cumulative curve is dominated by a two-section pattern (larger proportion of saltation section) and is partly represented as a three-section pattern (with small development scale of rolling section). The well logging curve is bell-shaped, box-shaped or a composite bell-box (Fig. 3a). Different from the conventional deltas, this shallow delta has shallow and torrent flow in the topset layer, and the lateral water body easily migrates and oscillates; the hydrodynamic condition is mainly veinous traction flow, with a small part of divergent runoff. The distributary channel is both the uploading terminal of sedimentation and the bypassing scouring channel of sandbody Natural levee Fig. 1 Structural unit division in the Sunda Basin By using core data and granulometry, drilling, well logging, mud logging and other data, it was determined that the shallow delta developed in the upper part of the Oligocene was dominated by a topset sandbody (delta plain subfacies), The natural levees are distributed on both sides of the distributary channel, which were formed by the siltation of seasonal stream during flooding period; the lithologies are mainly siltstone and silty clay rock, which gradually become smaller and thinner on both sides of the channel, with the development of parallel bedding and wavy cross bedding (Fig. 2b). Plant debris, root, burrowing, and the likes can be observed. The particle size probability cumulative curve is dominated by a three-section pattern (the saltation

3 Jiang Hui, The depositional characteristics and distribution law of shallow-water delta during late rifting stage... 3 section is smaller). The well logging curve appears as a finger or serrated feature (Fig. 3b). In the shallow water environment, the seasonal meandering river and the braided river were diverted into the lacustrine frequently. The friction of the riverbed reduces the kinetic energy of water body and lowers the loading capacity. It can be developed into a number of sand-rich natural levees with continuous migration and accumulation. Fig. 2 Sedimentary microfacies of typical cores in the Sunda Basin (a) Distributary channel, low-angle tabular cross bedding, Well D7, ft; (b) Natural levee, parallel bedding and wavy cross bedding, Well B4, ft; (c) Interdistributary bay, parallel bedding, with more argillaceous bands, Well B6, ft; (d) Back bank swamp, massive bedding, with plant debris, Well C2, ft; (e) Prodelta, parallel bedding, Well C2, ft Fig. 3 Templates of electrofacies of sedimentation in the Sunda Basin (a) Distributary channel (Well D7), (b) Natural levee (Well B4), (c) Interdistributary bay (B6 well), (d) Back bank swamp (Well C2), (e) Prodelta (Well C2) 1.3. Interdistributary bay The interdistributary bay mainly exists in the low depression of the middle of the distributary channel and is connected with the lacustrine. The lithology is mainly composed of argillaceous sediments, containing a small amount of thin layered or lenticular silty siltstone interlayers, with well-developed parallel bedding or horizontal bedding ( Fig. 2c); a series of wedge-shaped argillaceous sediments whose tips point to land, commonly known as mud wedge, are easily formed on the side to the lacustrine, there is more plant debris, and bioturbation is also strong with fossil organisms. The particle size probability cumulative curve is dominated by a two-section pattern (more suspension section); the log curve is similar to the mudstone section, being smooth or microscopically serrated (Fig. 3c) Back-bank swamp The back-bank swamp is a low-lying area with a large number of periodic flooding on the outside of the distributary channel. The plant is luxuriant and belongs to a weak reducing environment. The lithology is mainly dark organic mudstone, peat or lignite deposits, being interbedded with thin siltstone layers deposited by flood. There is massive bedding and horizontal bedding (Fig. 2d), with strong bioturbation and burrows. It contains plant debris, plant root, ostracod, siderite, and the likes. The particle size probability cumulative curve is dominated by a two-section pattern (large proportion of suspension section); the well logging curve is mainly microscopically serrated (Fig. 3d). The back-bank swamp in the shallow delta was more developed than that in conventional delta. During the rising period of lacustrine level, when the progradation rate of delta was lower than the rising speed of lacustrine level, the delta lobe was gradually abandoned, turned swampy quickly, and then became an important coal gathering place [23 25]. During the descending period of lacustrine level, the incision of the river was incised, and the abandoned delta lobe was reworked- or there was progradation of newly generated lobe in the pattern of spray fan [26 27] Delta front and prodelta The sandbodies in each of the microfacies of the delta front of the Sunda Basin are likely to be insufficiently de-

4 4 CHINA PETROLEUM EXPLORATION Vol. 23, No. 5, 2018 posited due to periodic exposure to the surface of water level fluctuation. They have limited developmental ranges and small longitudinal thickness, and are intertwined with the prodelta, which is dominated by mud and silty mud deposits. Parallel bedding, horizontal bedding, or massive bedding (Fig. 2e) can be observed with the development of a variety of bioturbation structures and benthic burrows. The particle size probability cumulative curve is a two-section pattern (large proportion of suspension section). The well logging curve is smooth or microscopically serrated (Fig. 3e). 2. Sedimentary distribution pattern of shallow delta During the late rifting stage of the Sunda Basin, the subsidence rate was significantly reduced, and the strata were strongly denudated; the climate was relatively humid, and the provenance supply was sufficient. Two shallow deltas were developed in the western slope and southern uplift of the basin (Fig. 4). The delta developed in the NW-striking short axis was larger, and three major distributary channels extended farther in the lacustrine basin and advanced eastward to the North sag, the Yani sag, and the Rosalia sag, which were in serrated and banded distributions on plane. The sandbody of the istributary channel varies rapidly in the lateral direction, and has the characteristics of a partial meandering river. The middle part was intercalated by massive muddy interdistributary bay. The outer sides of river channel were developed with a sand-rich natural levees, with sandbody thickness of 7 10 m and an average sand content up to 52.5%. Fig. 4 Sedimentary facies map during late rifting stage of the Sunda Basin The delta developed in the southern long axis was smaller and was mainly distributed in the Zelda sag. It mainly represents lobe protrusion on plane, reflecting that the provenance supply was more abundant and the forward movement of the fluvial sandbody was relatively faster during the Late Oligocene. The delta was continuously pro-

5 Jiang Hui, The depositional characteristics and distribution law of shallow-water delta during late rifting stage... 5 graded to the hinterland of the basin in long axis direction. The natural levees were mainly distributed on the west side of the Zelda Sag, with a sandbody thickness of 8 15 m and an average sand content up to 61.3%. Due to the higher content of sand and relatively stable distribution of natural levees, lateral accretion of distributary channel was restricted, and the crevasse fan was not developed. Back-bank swamps were widely distributed in the outer sides of the two shallow deltas, occupying a large area of the basin. In the inner sides, irregularly-shaped interdistributary bays were formed. The distribution of swamps in shallow deltas was more extensive. The reason is that on one hand, the outer edge of distributary channel is more likely to be fouled and deposited in shallow water to form a natural levee, and the distributary channel and sand-rich natural levee sandbody were continuously promoted, thus the rivers gradually exposed above the water surface to form a swamp; on the other hand, some sedimentary microfacies (such as delta front underwater distributary channel, underwater natural levee, estuary dam) being newly formed during lacustrine level rising filled the abandoned delta lobe to be gradually swamped with the fluctuation of lacustrine level when the lacustrine level dropped again. During the late rifting stage of the Sunda Basin, the climate was mild and humid. Influenced by the overall atrophy of fault activity in the basin and weakening activity of secondary faults associated with basin-controlling faults, the basin filling was dominated by deposits from seasonal traction flows. Due to the feature of shallow basin, gentle slope and rapid current, the delta sandbody advanced into the basin more quickly, and some distributary channels and natural levees might have extended to subaqueous areas and become underwater natural levees and underwater distributary channels. There is another viewpoint [28] in which the sandbodies in each of the microfacies of the delta front were actually formed by filling when there was descending of water level, exposure of fluvial bottom riverbed, and fluvial incision; they periodically submerged in the water when the water level rose. According to this cognition, the underwater distributary channel and the underwater natural levee during the late rifting / depression stages were only the non-synchronous products of the distributary channel of delta plains in low system tracts and of natural levees in high system tracts. In other words, delta front sandbodies in this shallow delta were not very developed, coupled with the prodelta, and the deposits dominated by mud were concentrated in the eastern part of the basin. This directly resulted in worse developed gravity flow depositions (such as slumping turbidite fan) in this area. 3. Deposition differences from conventional delta The shallow delta during the late rifting stage of the Sunda Basin was significantly different from other large shallow deltas and conventional deltas (Table 1), mainly including the following aspects: (1) the sedimentary configuration was controlled by shallow surface runoff traction into the lacustrine hydrodynamic environment with gentle slope and shallow water. The trend of distributary channel or natural levee shaped the plane distribution pattern of sandbody and the section morphology of sandbody was also Table 1 Simplified table for comparison between shallow delta during late rifting stage and large shallow deltas and conventional deltas Sedimentary type Shallow delta during late rifting stage Large shallow delta Conventional delta Plane morphology Serrated sheet-shaped, lobe-shaped, serrated, branched, and banded, etc. Bottomset and foreset may be not completely developed. Extremely shallow water, offshore lacustrine (marine) Divergent runoff and venous traction flow dominated by seasonal meandering river and braided river. Inertia force of water, turbulence force, riverbed friction, air friction on river surface Digitate, beak-shaped, lobe-shaped, and branched, etc. Topset, foreset and bottomset may be not completely developed. Shallow water, offshore lacustrine (marine) Meandering river, braided river, or anastomosing river, extensive surface runoff and tubular traction flow Inertia force of water, riverbed friction, gravity of weak terrain slope, air friction on river surface, and air dynamics Distributary channel, underwater distributary channel, natural levee, estuary bar, sand sheet Alternately influenced by thermal diffusion and condensation of hydrone Digitate, beak-shaped, lobe-shaped, embayed, etc. Triple structures (topset, foreset, and bottomset) Normal water depth, semi-deep lacustrine (marine), deep lacustrine (marine) Meandering river, braided river, or anastomosing river, extensive intensive traction flow Inertia force of water, riverbed friction, gravity dynamics of weak terrain slope, air friction on wide river surface, wave and tidal actions Distributary channel, underwater distributary channel, distal sand bar, estuary bar, sand sheet Vertical structure Environment of water body Primary provenance supply Dynamic mechanism of estuary area Microfacies of sand-rich deposits Distributary channel, natural levee Influence of climate Prominently controlled by seasonal warm and humid climate Weakly influenced by climate

6 6 CHINA PETROLEUM EXPLORATION Vol. 23, No. 5, 2018 more diversified. The foreset and the part below were gradually swamped due to periodic exposure and filling, therefore the back-bank swamp was more developed and coal measures were more widely distributed compared with a conventional delta. (2) The dynamic mechanism of the estuary area mainly consists of the hydrodynamics of divergent runoff and vein traction flow, the friction of riverbed, and the air friction on the river surface; the gravity driving force caused by terrain slope and the wave and tidal actions were weaker. The shallow water column especially caused the mechanism of gravity deposition to not be well developed. (3) The sand-rich microfacies of the shallow delta during the late rifting stage of the Sunda Basin were mainly distributary channels and natural levees. The distributary channels are both sedimentary unloading terminals and passing scouring channels of sandbodies. The riverbed friction reduced the kinetic energy of water, and the loading capacity was lowered, thereby forming a number of small inter-river sand bars due to the deposition of sandbodies. As sediments began to accumulate upward due to the migration and swaying of sediments, it caused the original inter-channel sand bar to be gradually annihilated and gradually develop into a sand-rich natural levee. This is also the difference between such shallow deltas and other large shallow deltas and conventional delta deposits. (4) Climate is one of the primary factors affecting the shape of sandbodies in the ashallow delta. Under drought conditions, the influence of thermal diffusion of water molecules is enhanced, and the waters become shallower as the result of shrinking lacustrine. Seasonal rivers carry debris material for a long distance, forming serrated sheet and lobe-shaped shallow deltas dominated by a delta plain. Under humid conditions, the expansion and condensation of lacustrine are enhanced; being affected by the topping action of lacustrine water, the channels frequently bifurcate and divert, forming serrated, banded, or branched shallow deltas dominated by a delta plain and delta front, which is particularly obvious in shallow delta during the late rifting/depression stage. 4. Initial cognitions The shallow delta during the late rifting stage has opened a new field for the exploration of oil and gas reservoirs (especially subtle lithologic reservoirs). The continental strata in China have experienced multiple stages of tectonic cycles and thermal evolution. Overlapping rift valleys or smallmedium rift lake basins were widely distributed spatially and temporarily, providing a broad possibility for the emergence of shallow deltas and ultra-shallow deltas. The significance of identifying shallow deltas during the late rifting stage is that its sedimentary characteristics are different from those of fluvial deposits, and different from those of conventional deltas and other large shallow delta deposits. The facies combination is dominated by delta plains, and the vertical facies sequence is often incomplete. Distributary channels have no obvious incision, but obvious bifurcation, high sheet-shaped degree of sandbodies, and good lateral connectivity. The sand-rich natural levees are well-developed directionally, which can act as high-quality reservoirs. The back-swamps are extremely developed, usually forming caprocks with good configuration. In addition, strike-slip faults or the associated faults with strike-slipping nature were often developed during the late rifting stage of this basin, which is conducive to the formation of a variety of lithologic traps and construction of migration channels for hydrocarbon. 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