A series of Neogene lithologic reservoirs have been discovered in the Dongying Sag of the Jiyang Depression, Bozhong Sag of Bozhong Depression and Qingshui Sag of Liaohe Depression, Bohai Bay Basin^[1,2,3]. We have got some understandings on accumulation factors, sedimentary environments, reservoir characteristics and hydrocarbon migration model of the Neogene lithologic reservoirs^{[4,5,6,7,8,9,10,11,12,13]}. In 2002, we proposed for the first time that a Neogene lacustrine-delta system was developed in the Bozhong Depression^[8]. In 2006, Wang et al.^[10] suggested lithologic or stratigraphic overlap traps developed in the braided river deposits in the Neogene Guantao Formation (N₁g) of the Jiyang Depression controlled by paleo-geomorphology. With regard to the Neogene lithologic hydrocarbon migration model, Zhang et al. and Deng et al.^[11,12] proposed "meshwork-carpet" and "transfer station" hydrocarbon migration models, respectively. Xue^[13] pointed out that the development of "structural ridge" in the Paleogene of Bohai Sea was the key factor affecting hydrocarbon accumulation in favorable traps of the overlying Neogene. These theories provided references for the exploration in this area.

As the Laibei Low Uplift (LLU) in the Bohai Sea has special petroleum geologic conditions, two obstacles hold back the lithologic reservoir exploration there: (1) The sedimentary environment of this area was quite different from the Neogene shallow-water delta of the Lower Member of Minghuazhen Formation (N₂m^l) in the Huanghekou Sag, or the braided river superimposed contiguous sand bodies of the N₁g in the Bohai Sea. No widely-distributed sand bodies had discovered in this area and the conventional view was that there were no large-scale lithologic traps. (2) It was uncertain whether the LLU, submerged at first and uplifted afterwards, could receive large amount and long-term supply of oil and gas to form high-abundance lithologic reservoirs. We must work hard to unravel these questions to seek a breakthrough in exploration of large high-abundance lithologic oilfields in the study area.

After several years of hard search, the KL6-1 oilfield, a 100-million-ton reserve-scale high-abundance lithologic reservoir, was discovered in the LLU, ending the history of no large oilfield in the LLU. Based on the drilling data of more than 30 wells, analysis data of over 300 samples and 3D seismic data of 800 km², basic characteristics of the Neogene large-scale and high-abundance lithologic reservoir in the LLU have been sorted, the oil source, genetic mechanism of the reservoir, and the hydrocarbon migration mode have been examined, in the hope to provide references for the future exploration of lithologic reservoirs in the Bohai Sea or other areas with similar geological backgrounds.

The LLU is a rhomboid area striking northeast in the southern Bohai Sea surrounded by the eastern branch of the Tanlu strike-slip fault (TSF), the western branch of the TSF, the Laizhouwan Sag in the south, and the Huanghekou Sag in the north, with an area of 1050 km² (Fig. 1). It is covered by the Paleogene Shahejie Formation (E₂s₃, E₂s₂, E₂s₁), the Paleogene Dongying Formation (E₃d₃, E₃d₂, E₃d₁), the Neogene Guantao Formation (N₁g), the Neogene Minghuazhen Formation (N₂m^l, N₂m^u), and the Quaternary Pingyuan Formation (Qp). It was submerged underwater in the early stage and uplifted later.

The northern and southern boundaries of the LLU are defined by extensional faults, while the eastern and western boundaries are defined by the TSF^[14]. The basement of the LLU is a long and narrow, NE-SW striking low uplift. Its northeastern slope and southwestern slope are connected to the Huanghekou Sag and the northeastern Laizhouwan Sag, respectively (Fig. 2a).

Influenced by multi-stage superposed strike-slip and extension, a series of extension faults or extension faults derived from strike-slip developed on the LLU (Fig. 2b). One group of the faults are a series of large-scale nearly EW trending extensional faults, which had strong control on the Paleogene deposition. Faults of this group are plough-like normal faults on the section with strike-slip features, which constitute negative flower structures with secondary faults at shallow depth. Extensional faults in NE strike of the other group occurred near the middle branch of strike-slip belt and the slope belt extending into the Huanghekou Sag. These faults are slab or plough-like on section and mostly cut the Paleogene and even down to the basement.

From the Paleocene to Middle Eocene, NS-trending extension dominated the study area, resulting in a series of EW extension faults. The LLU, located on the upthrow block of the Laibei No.1 fault, was uplifted and denudated. From the Middle-Late Eocene to the Late Oligocene, as the dextral strike-slip movements of the TSF gradually enhanced, the LLU held between the eastern and middle branches of the TSF rotated clockwise passively. As a result, the eastern part of the study area experienced uplifting in the north and subsiding in the south, while the western part of the study area experienced uplifting in the south and subsiding in the north. These tectonic movements established the long and narrow, NE strike rhombus shape of the LLU. In the Miocene, affected by lithosphere equilibrium and crustal cooling and contraction, the entire Bohai Bay Basin subsided, and the sedimentary center gradually migrated to the northern Bozhong Sag, while the LLU became the marginal slope zone of the sedimentary center^[12].

The LLU is different from other uplifts in Bohai Sea in strata and sedimentary environment evolution^{[15,16,17,18]}. From the bottom to the top, the Cenozoic strata of the LLU consist of E₂s₃, E₂s₂, E₂s₁, E₃d₃, E₃d₂, N₁g, N₂m^l, N₂m^u and Qp. Paleocene stratum, local E₃d₁, and E₂s₃ are absent in LLU (Fig. 3).

During the sedimentary period of E₂s₃, the LLU submerged and accepted sediments from the Kendong Uplift in the southwest, developing braided river delta and shallow lacustrine deposits of siltstone, fine sandstone, medium sandstone and thick brown grey mudstone. At the end of the sedimentary period of E₂s₃, the LLU was exposed and the braided-river-delta deposits were denudated. During the sedimentary period of E₂s₂-E₂s₁, the LLU submerged again, developing shallow lacustrine and braided river delta deposits of light gray fine sandstone interbedded with gray mudstone. During the deposition of E₃d₃ and E₃d₂, the LLU inherited the previous structural pattern, developing shallow lacustrine and braided river delta deposits of interbeds of light gray sandstone and gray mudstone, with some green gray tuff and basalt interlayers in the upper part. At the end of the sedimentation of E₃d, affected by the Dongying Movement, the study area was structurally inversed, uplifted and denudated, resulting in the missing of E₃d₁. In the Neogene, the Bohai Bay Basin entered the post-rift depression stage. During the sedimentation of N₁g, the study area experienced intense planarization, developing braided river deposits of thick sandstone with mudstone interlayers. During the sedimentation of N₂m¹, the LLU was characterized by extremely gentle slope, limited accommodation space and frequent switch between fluvial and lacustrine environments, developing shallow water delta deposits of mudstone, fine sandstone and siltstone (Fig. 4).

According to the regional stratigraphic correlation, oil-water distribution and strata thickness, the N₂m^l of the study area is divided into five oil layer groups (I-V oil layer group). Of them, the III-V oil layer groups are interbeds of sandstone and mudstone, with superior reservoir-seal combinations. The III-V oil layer groups are the main oil-bearing intervals in the study area, the V oil layer group of which is the main oil-bearing layer. In this work, the high-resolution sequence stratigraphy of the III-V oil layer groups is introduced.

A complete medium-term base level cycle, SQ₁, developed in the middle and lower part of the V oil layer group in N₂m^l. During this period, the LLU was an extremely gentle slope with limited accommodation space and sufficient sediment supply from the northeast. The mudstone of this section was brownish red, indicating oxidizing environment (Fig. 5). A complete medium-term base level cycle, SQ₂, developed in the upper part of the V oil layer group and the III oil layer group. Since the late depositional stage of the V oil layer group, the base level rose rapidly, the accommodation space increased, and a set of widespread sand bodies deposited, which becomes the main reservoir of KL6-1 oilfield. Afterwards, the water deepened continuously, the mudstone changed from brownish red to gray green, and the accommodation space reached its maximum in the middle part of the IV oil layer group, and the abundance and diversity of algae also reached the maximum, indicating the area was shallow lacustrine environment. Subsequently, the base level dropped and the mudstone gradually turned to reddish brown, indicating lake water retreat, leaving the sand bodies to fluvial process.

The KL6-1 oilfield, a large-scale lithologic oilfield, was finally discovered after years of continuous research on the LLU. The following part is a detailed introduction of the physical properties of the oil layer group, oil-water relationship, oil properties, tested production capacity, temperature and pressure conditions, reservoir type, reserve scale and other characteristics of this oilfield.

The KL6-1 oilfield is a monolithic, high-quality, large- scale Neogene lithologic oilfield with the characteristics of shallow burial depth, high test productivity, concentrated oil-bearing intervals, large oil-bearing area, high reserves abundance, and high oil quality.

The oil layers of KL6-1 oilfield are distributed in the Neogene N₂m^l. According to the correlation relationship of reservoirs, the N₂m^l is divided into five oil layer groups I, II, III, IV and V from top to bottom. Although oil and gas layers are distributed in all five oil layer groups, the main oil-bearing intervals are mainly concentrated in the middle and upper part of V oil layer group and the bottom of the IV oil layer group, with a burial depth of 1200-1550 m. The sand bodies are stable in thickness, about 3-15 m according to drilling data, and 10 m on average (Fig. 6).

The reservoir rock is medium-fine lithic feldspar sandstone, composed of mainly quartz, feldspar and debris. The clastic particles in the reservoirs are moderate-well sorted and subangular-subrounded. The reservoirs have a porosity range from 15% to 40%, on average 31%, a permeability range of (8-19 721)×10^-3 μm², on average of 2205×10^-3 μm², and are mostly high porosity and high permeability ones. Pores in the reservoirs are mainly primary inter-granular pores, abundant, evenly distributed and well-connected. The intergranular filling material is filiform lamellae illite and montmorillonite mixed layer. The particles are in point contact, and the feldspar is moderately weathered (Fig. 7).

The KL6-1 oil reservoir, with a single oil-bearing series and a uniform oil-water interface for each sand body, is an edge water reservoir. The main oil/gas-bearing interval of the KL6-1 oil reservoir has temperatures of 50.0-72.7 °C, a temperature gradient of 3.56 °C/100 m, pressure coefficient of 1.02, and pressure gradient of 0.916 MPa/ 100 m, representing normal temperature and pressure system. Located in a low uplift area, the KL6-1 reservoir has oil and gas in layer distribution along widespread contiguous sand bodies. The distribution range of the sand bodies determines the oil-bearing area (about 100 km²). The filling capacity of oil and gas determines the hydrocarbon column height (about 40-80 m). The reservoirs are typical lithologic and structural-lithologic ones.

The crude oil of KL6-1 reservoir is fairly high in quality, with medium-average density, high viscosity, medium colloid asphalt content, medium wax content, low sulfur content, and high freezing point. The surface crude oil has a density of 0.906-0.939 g/cm³ (20 °C), viscosity of 43.36-113.80 mPa·s (50 °C), sulfur content of 0.21%-0.37%, wax content of 3.04%-16.01%, colloid and asphalt content of 13.27%-22.63%, and freezing point of 16.0-27.0 °C. Thanks to the good physical properties of the crude oil, the wells had a tested daily productivity of 79-187 m³.

With a superposed area of main oil layers of over 100 km², the KL6-1 oilfield has a large reserve scale, with proven oil reserves at the order of 100 million tons and reserve abundance of about 110×10⁴ m³/km². The KL6-1 oilfield is a monolithic, high-quality, large-scale Neogene lithologic oilfield with reserves at 100-million-ton order in Bohai Bay Basin.

The KL6-1 lithologic oilfield, located on the LLU, is adjacent to two sags with abundant hydrocarbon: the Huanghekou Sag and the Laizhouwan Sag, so it has superior oil hydrocarbon supply from sources in two directions (Fig. 8).

There are three sets of hydrocarbon source rocks developing in the Huanghekou Sag, E₂s₃, E₂s₂ and E₂s₁, and E₃d₃. The source rocks are mainly dark gray, gray black and black mudstone and oil shale. Statistics show that the E₂s₃ and E₃d₃ source rocks are good-excellent in quality; the E₂s₁ and E₂s₂ source rocks are medium-good in quality. They all have mainly II₁-II₂ types of organic matter (Table 1). The source rocks of E₂s₃, E₂s₁ and E₂s₂ are higher in maturity and in the peak period of hydrocarbon generation. E₃d₃, shallow in burial depth, has just matured^[19]. According to the comparative analysis of hydrocarbon generation indexes and oil-source correlation of each layer, it is considered that the hydrocarbon source rock of E₂s₃ is the most favorable hydrocarbon source rock in the Huanghekou Sag^[20]. The hydrocarbon source rock of E₂s₃ is characterized by medium diasterane, medium gammacerane and medium tetramethylsterane contents (Fig. 9a).

There are two sets of hydrocarbon source rocks developing in the Laizhouwan Sag, involving E₂s₄ and E₂s₃ hydrocarbon source rocks^[21]. They’re mainly thick dark mudstone with basalt or sandstone in local parts. The statistics on organic matter abundance and type parameters show that both the E₂s₄ and E₂s₃ hydrocarbon source rocks are good-excellent. E₂s₃ source rock mainly has II₁-III type organic matter, and E₂s₄ mainly has II₁-II₂ type organic matter (Table 1). The E₂s₄ hydrocarbon source rock is mostly in the peak of hydrocarbon generation, while the E₂s₃ hydrocarbon source rock has mostly matured but with lower maturity, and only entered peak of hydrocarbon generation in local parts. Different from the E₂s₃ hydrocarbon source rock of the Huanghekou Sag, the hydrocarbon source rocks of E₂s₃ and E₂s₄ in the Laizhouwan Sag are characterized by high diasterane, low gammacerane and high tetramethylsterane contents (Fig. 9b).

The GC-MS analysis of saturated hydrocarbons shows that the KL6-1 reservoir is characterized by hydrocarbon sources from two sags. The crude oil in the main area of KL6 is characterized by medium-high diasterane, medium gammacerane, medium-low tetramethylsterane contents, Ts/Tm>1 and medium maturity, which is consistent with the E₂s₃ hydrocarbon source rock of the Huanghekou Sag (Fig. 9a, c), indicating that the crude oil in the main area is mainly from the Huanghekou Sag. In contrast, the crude oil in the KL10-1N block near Laizhouwan Sag is characterized by high diasterane, low gammacerane, high tetramethylsterane contents, Ts/Tm>1 and medium-high maturity, which is consistent with the characteristics of E₂s₃ and E₂s₄ hydrocarbon source rocks in the Laizhouwan Sag (Fig. 9b, d), indicating that the hydrocarbon source rocks in the Laizhouwan Sag make some contribution to KL6-1 oilfield.

3.2.1. Development of two types of "inherited convergence ridges" due to block rotation

According to earlier studies on the regional tectonics, the structural evolution of the LLU controlled by the TSF and mantle upwelling is characterized by multiple genetic mechanisms^{[22,23,24,25,26,27]}. In the early Eocene, due to the N-S extension, several domino-type tilted fault blocks developed in the southeast of Bohai Sea, and the prototype of the LLU initially came up. From the end of Eocene to Oligocene, as the dextral strike-slip activity of the TSF strengthened, the N-S extension and N-NE shearing jointly controlled the tectonic evolution and geometry of the LLU. During this period, under the joint effect of the eastern and middle branch of the TSF and the southern boundary fault, the LLU rotated clockwise^{[14, 27-28]}. The block rotation made the northwestern and southeastern parts of the LLU in the extensional stress zone far away from the strike slip fault zone, resulting in local subsidence, whereas the northeastern and southwestern parts of the LLU stayed in the compression stress area, characterized by local compression and uplifting. In Neogene, as the whole Bohai Bay Basin subsided, entering the depression stage, the overall structural pattern of the LLU and its surrounding area was relatively stable too.

Because of the block rotation, two types of inherited structural ridges developed in the KL10-1 block in the southwestern LLU and in the KL6 block in the northeastern LLU (Fig. 10). For the two highly permeable unconformities, T₈ developing before Cenozoic and T₅ developing at the end of Paleocene, these two types of structural ridges are very important oil and gas “structural ridges” which provide large-scale secondary migration conditions to the Neogene shallow layers^[29].

Among them, the "structural ridge" of the KL10-1N structure is a "fault anticline structural ridge" with elevation rising from north to south and high part blocked by the southern boundary fault. The "structural ridge" of the KL6 block is a typical "anticline structural ridge" high in the middle and lower on both wings. These two types of high-quality structural ridges provide favorable conditions for oil and gas migration and secondary hydrocarbon source supply for the LLU with no hydrocarbon generation conditions itself, laying a material foundation for the accumulation of large oil and gas reservoirs far away from oil source areas. The drilling results have also confirmed that there is a good spatial matching relationship between the oil-bearing range of Neogene and the distribution of structural ridges on the LLU.

3.2.2. The late active faults resulting from the neotectonic movement on the LLU

Since the Neogene, the Bohai Bay Basin remained relatively stable during the sedimentation of N₁g became active again during the sedimentation N₂m^l affected by the tensional-shear stress due to remote effect of Pacific Plate subducting towards the Eurasian Plate and the Indian Plate colliding with the Eurasian Plate^[30,31,32] and was in tensional-torsional stress field. The E-W extension faults in the LLU reactivated, and showed left-lateral strike-slip property. A large number of shallow NE faults turned up with the E-W extension faults, forming complex 'Y' shape or negative flower shape structures. The E-W extensional faults are larger in scale and more active (15 m/Ma), many of which cut down to the sea floor and associate with natural gas seeps, indicating that the faults are still active and serve as main migration faults in the study area.

The LLU, located between the middle branch and the eastern branch of the TSL, was under the influence of NNW extension and NE-SW compression jointly, so a series of NE trending extensional faults in en echelon formation turned up on the northern slope. Some of them cut down to the basement, effectively connecting the deep structural ridges with the shallow reservoirs, and becoming favorable oil and gas transportation paths.

3.2.3. The widespread overlapped sand bodies

At the end of the deposition of the V oil layer group in N₂m^l, the LLU was a gentle slope frequently switching between fluvial and lacustrine environments, where a set of lateral stable shallow water delta front sand bodies developed. Because of the very small gradient of the area, distributary channels crisscrossed like a net, connecting and overlapping with each other. The lake waves reformed the distributary channel sand bodies into thin sheet contiguous in distribution, filling between the distributary channels. Eventually, overlapped contiguous sand bodies over a large area turned up.

3.2.4. Efficient hydrocarbon transport network composed of structural ridges, late active faults and widespread contiguous sand bodies

The two types of inherited structural ridges, late active faults connecting the structural ridges, and widespread contiguous sand bodies constitute the efficient hydrocarbon transport network of the LLU. In the initial stage of exploration, the KL4-1 structure on the northwestern slope of the LLU was selected as the main exploration target. This structure is located in the northwest slope of the LLU, with abundant NW trending extensional faults. However, as this structure is not located on the inherited structural ridge (Fig. 10) and the sand bodies of the IV-V oil layer groups in N₂m^l are isolated with poor lateral connection, there is no efficient oil and gas transportation network system in this area, making it impossible for the oil and gas from the Huanghekou Sag to migrate to and accumulate in the shallow sand bodies there. As a result, the exploration wells in this area were low in success rate (50%), and the discovered reserves are small in scale or scattered, with no commercial development value. The KL6-1 structure, on the other hand, is located on the anticline structural ridge of the LLU (Fig. 10), where widespread contiguous lowstand sand bodies of N₂m^l and NE trending extensional faults cutting down to the basement to connect the structural ridge with the lowstand sand bodies of N₂m^l constitute an efficient oil and gas transport network, serving as smooth migration pathways. Therefore, this area has oil and gas in more concentrated distribution and higher reserve abundance, making it the major production area of the KL6-1 oilfield.

3.3.1. Sedimentary characteristics of oil layer groups and the origin of overlapped sand bodies in the Lower Minghuazhen Member

The oil layers in the N₂m^l are very stable in distribution vertically. About 80% of the oil layers are distributed at the early stage of ascending semi-cycle of the SQ2 base level (top of the V oil layer group). During this stage, the base level and accommodation space were low, and transverse overlapped lowstand shallow water delta sand bodies stable laterally developed, which are named lowstand system sand bodies (Fig. 11).

In middle late Miocene, the LLU experienced uplifting and denudation in Oligocene, filling and leveling during the depositional stage of N₁g, and finally became a relatively gentle slope to the depositional stage of N₂m^l and N₂m^u. The slope was less than 0.5° according to the lateral change rate of the stratum thickness. Some places subsided more, forming shallow lake, while the study area developed mainly shallow water deltas.

As the thickness of the V oil layer group of N₂m^l changes little, it is inferred that during the sedimentation, this area was very gentle in topography (with a slope < 0.5°). Therefore, a weak fluctuation of the lake surface could result in large migration of the lakeshore, and wide shallow water delta front facies developed. The shallow water delta front can be divided into inner front and outer front. The inner front refers to the area between the average high water level and the average low water level. The outer front refers to the area below the average low water level. The inner front area, intermittently covered by lake water most frequently, would more likely be subjected to frequent scouring of lake water. Lowstand system sand bodies occur mostly in the inner front subfacies. Based on observation of cores from 3 wells, particle size probability curves and plane attribute characteristics, we found the shallow water delta inner front area developed underwater distributary channel, inter-distributary, river mouth bar and sheet sand body etc microfacies. The underwater distributary channel is the extended river channel below the lake surface. A whole underwater distributary channel sequence is a finning-upward sequence, with fine sandstone containing argillaceous gravels at the bottom and thin interbeds of fine sandstone and mudstone at the top, and oblique beddings and plate-like cross beddings. The river mouth bar facies, located at the end of the underwater distributary channel, is coarsening-upward sequence of siltstone and fine sandstone, with horizontal beddings, which takes on funnel-shape on logging curves. The sheet sand, usually occurring at the front of river mouth bar or between the distributary channels, is the product of re-deposition of mouth bar and underwater distributary channel sand under the reformation of hydrodynamic forces such as wave and alongshore current. The sheet sand mainly consists of siltstone, with mud strips in local parts (Fig. 12).

In the dry season, the lakeshore was low and the inner front was exposed, and dense net-like distributary river channels developed. The distributary channel sand bodies are thinner than typical meandering river sand bodies and 8-12 m thick in general. In flood season, the lake level rose and the shoreline shifted towards inland. The dense distributary channels were submerged and reformed by wave and along shore current, turning sheet-like. The thin sheet sand bodies connected the distributary channels, forming large-scale overlapped sand bodies. 40 wells drilled targeting the lowstand system sand bodies have confirmed that the lowstand system sand bodies are connected laterally in large scale and share the same oil-water contact and pressure system. After the sedimentation of the lowstand system sand bodies (the top of the V oil layer group of N₂m^l), the rapid rise of the base level led to transgression, which might be related to the enhancement of the Asia summer monsoon in Late Miocene (8.5-6.0 Ma)^[33,34]. During the transgression period, mudstone was deposited directly on top of the lowstand sandstone, forming a favorable reservoir- cap rock assemblage (Fig. 13).

3.3.2. Lithologic traps in limited accommodation space

During the sedimentation of the Ⅲ-V oil layer groups of N₂m^l on LLU, the study area was characterized by gentle slope, shallow water and limited accommodation space, moreover, the base level changed frequently due to the effects of multiple factors such as climate, season and hydrodynamic forces, so overlapped shallow water delta and meandering river sand bodies developed, forming two types of lithologic traps, lithologic pinch-out trap and fault-lithology composite trap (Fig. 14).

The lithologic pinch-out trap includes delta front lithologic pin-out trap and paleo-channel lithologic trap. During the rising semi-cycle of the base level, the study area frequently switched between fluvial environment and lacustrine environment, thus the distributary channels and mouth bars of the shallow water delta front were frequently reformed, consequently, delta front sand bodies were likely to be wrapped by impermeable layers like mudstone layers, forming delta front lithologic pin-out traps. Traps of this type occur most commonly in the KL6/5 structure, and are often contiguous and cut by faults. The paleo-channel lithologic traps usually developed at the late stage of the descending semi-cycle of the base level, which were made up of meandering river channel or shallow delta distributary channel sand bodies in strip shape blocked by adjacent impermeable layers. The KL10-1N structure is a trap of this type.

The fault-lithology composite trap refers to the composite trap with sand bodies cut by faults. The fault terrace zone of the KL6-1 structure was this type of trap. Reformed by lake water, the sand bodies were sheet-like and widespread. These sand bodies were cut and blocked by late-stage faults, forming fault-lithology composite traps.

The inherited structural ridges, active oil source faults and widely distributed sand bodies give rise to the "vine type" hydrocarbon migration and accumulation model of LLU, which is the key to the formation of large area and high-abundance lithologic reservoir in this area. The “vine type” hydrocarbon migration and accumulation model includes three major reservoir forming factors (Fig. 15).

3.4.1. Deep oil and gas structural ridges

The oil and gas structural ridges formed during the reservoir-forming period by the T₈ and T₅ unconformities in the study area are effective transfer stations for oil and gas accumulation in N₂m^l[35,36]. The accumulation capacity of them determines the scale of oil reservoirs in the shallow sand bodies. The LLU has the KL10-1N “fault anticline structural ridge" and KL6 "anticlinal structural ridge" in the east and west segments of Paleogene respectively. The lithologic traps located above the structural ridges are favorable areas for oil and gas accumulation.

3.4.2. Faults: the preferential migration pathway

The activity of oil-source faults controls the vertical migration efficiency of oil and gas. After gathering in the preferential convergence area, oil and gas need migrate along faults to shallow layers. The stronger the fault activity is, the higher the efficiency of vertical oil and gas migration will be. Using dipole S-wave remote detection technology to identify and count the micro fractures of oil source faults, it is found that when the fault activity rate is greater than 15 m/Ma in the reservoir-forming period, micro fractures would be well-developed, and oil and gas can be found in shallow layers generally. The NE trending fault systems densely developing above the hydrocarbon structural ridges are preferential channels for hydrocarbon migration from the deep structural ridges to the sand bodies in N₂m^l.

3.4.3. The large "vine type" overlapped continuous sand bodies

The densely intersected and connected distributary channels of the N₂m^l lowstand delta front sand bodies are characterized by a netlike sand body skeleton, called the "vine" of the overlapped sand bodied. The distributary channel sand bodies were reworked by lake wave and redeposited between the channels forming sheet sand bodied with relatively thin thickness and plane continuous distribution, which called the “leaf” of the sand body. The distributary channel sand bodies are thick and feature good physical properties. When it contacts with the migration fault, it not only act as “highway” for hydrocarbon lateral migration, but also as the reservoir for accumulation. Along with the continuous charging, oil migrates from “vine” to “leaf” driven by buoyancy, and finally form the plane continuous oil distribution characteristics.

The exploration results show that the KL6-1 oilfield is a large high-quality lithologic oilfield, characterized by shallow reservoir depth, good oil quality, high test productivity, concentrated oil-bearing sections, large oil- bearing area and medium reserve abundance. Four geological factors are key for the formation of the KL6-1 oilfield: (1) The large overlapped continuous sand bodies depositing in the limited accommodation space of the IV-V oil layer groups of N₂m^l, with good reservoir physical properties and stable thickness, are one of the basic conditions for the formation the oil field. (2) The LLU, sandwiched between the Huanghekou Sag and the Laizhouwan Sag with rich oil and gas has the superior oil and gas supply from sources in two directions, providing material basis for the formation of large-scale lithologic reservoirs. (3) As a result of the block rotation caused by the activity of TSF and the southern boundary fault, there are two types of "structural ridges" (fault anticline and anticline) developing on LLU, providing favorable hydrocarbon accumulation conditions. (4) The three major reservoir forming factors, deep oil and gas structural ridges, preferential migration channels of faults, and large-scale "vine type" overlapped contiguous sand bodies together determine the hydrocarbon migration and accumulation model of the KL6-1 large lithologic oilfield, resulting in continuous distribution of oil and gas in a large area in N₂m^l.