Hydrocarbon Distribution and Accumulation Model in the South of Lixian Slope, Raoyang Subbasin

Lixian slope, lies in the western Raoyang subba-sin, is one of the most important oil-rich slopes in central Hebei depression. It is an NE strike wide-gentle successive sedimentary slope, dipping towards the east and uplifting in the west (Yang et al., 2010a, b; Zhou et al., 2009). The research area situates in the south of Lixian slope. It is bounded by Gaoyang, Xiliu nose structures in the east and west respectively, and Gaoyang faults in the north. The area has a simple structure and fewer faults developed. The most noti-ceable structures are NE-SW direction Dabaichi nose structure and Gaoyang nose structure, as well as many associated small nose structures (Fig. 1).

Figure 1.  Regional structure map of Lixian slope in Raoyang subbasin.

For decades, the search for oil and gas has been concentrated in the north region of Lixian slope; many patterns and models were established, and numerous reservoirs were found. However, the south part of Lixian slope remains stable for quite a long time without any major break though. The hydrocarbon accumulation mechanism and model for the south of Lixian slope are not fully understood. In the past, it was believed that hydrocarbon was "stepped-like" migration model, which means oil and gas generated from Es^1x and Es³ source rock in the inner slope transporting through small faults step by step towards outer slope high positions. But that model conflicts with newly discovered reservoirs, and seriously re-stricted exploration employment. Thence, hydrocar-bon accumulation model analysis is of vital impor-tance for reshaping the exploration situation of south Lixian slope.

Tectonic movements of Lixian slope have gone through several stages. The generation, development and collapsing of Raoyang subbasin was controlled by early Yanshanian movement and the following Hima-layan movement (Jin et al., 2007, 2004; Wang et al., 2002). There are 5 important tectonic movements during Paleogene that determines the sedimentary facies and paleogeomorphology of Lixian slope (Fig 2). Lixian slope is considered as a platform-gentle slope which comprises of platform and monocline from north to south (Wang et al., 2006). Fei and Wang (2004) suggests Lixian slope belongs to sedimentary slope according to the sedimentary features. From seismic profiles interpretation, the south part of Lixian slope is a successive developed slope that formed on the east flank of buried Gaoyang anticline. The struc-ture of the east flank is a gentle basement slope with-out apparent relief. Rennan fault that lies in the south-east is a boundary fault which has a great impact on Lixian slope. The tectonic activity was fairly weak during paleogene that made Rennan fault remained almost stationary (Yang et al., 2010c). Therefore the paleotopography in the south of Lixian slope is quite gentle, and far from the source kitchen in the south (Fig. 3).

Figure 2.  Stratigraphic column of Lixian slope.

Figure 3.  Seismic profile in the south of Lixian slope (A-A' profile as in show in Fig. 1).

The faults in the research area could be divided into two types based on basin evolutionary stage and seismic reflection. One is continuous developing type characterized of middle period growth, later period attenuation. This kind of faults is usually large scale developed, and mainly refers to Gaoyang fault and Dabaichi fault in the south of Lixian slope. They were generated during the beginning of the basin formed and died till the late Ng, but their growths were in ac-cordance with the Himalayan movement phases. They were not able to control the distribution of sand in paleogene and hydrocarbon migration, but determined the paleotopography after Ng. An other kind of faults is later period growth and late period attenuation. They were formed in the Late Oligocene (Es²–Ed), and continue to grow in Miocene or Pliocene (Ng–Nm). This kind of faults were usually small but numerous, NE and NEE direction, distributed mainly in the southeast of the slope. They had no obvious controlling effects on sedimentary and structure, but small structural traps and hydrocarbon secondary mi-gration.

There are 3 distinctive structural belts in Lixian slope that separated by faults and overlap zone. The outer slope and middle slope is divided by 2 big fault zones, Dabaichi fault and Gaoyang fault in the north. The middle slope and inner slope is separated by overlap zone in the south near Suning sag and Renxi sag (Fig. 1).

Through well cores observation, seismic attributes analysis and sedimentary parameters study, the Lixian slope is believed to develop shallow-water braided delta in Shahejie Formation (Wang et al., 2012; Cui et al., 2005). The abundant supply formed conti-nuous and stacked sand in the area. The favorable re-servoir is formed by distributed channel and distri-buted mouth bar. The porosity and permeability of the reservoir are 9%–24%, 0.1–52.7×10^-3 µm², respec-tively, and belongs to middle-low porosity and low permeability reservoir.

The strata of the south Lixian slope could be di-vided into 3 source-reservoir-seal assemblage accord-ing to the sedimentary cycle and mudstone (Fig. 2). The first assemblage is "lower generating-upper re-servoir" type which is composed of source rock from middle-lower Es³ (Es^3z–Es^3x), reservoir from upper Es³ (Es^3s), and seal from Es² "mud neck". The second type is "upper generating-lower reservoir" which is composed of "special lithologic section" in lower Es¹ (Es^1x) and its underlying bottom sand. The third type is "lower generating-upper reservoir" which is com-posed of Es^1x source rock, reservoir and seal from Ed and shallow strata (Wang et al., 2001).

There are two sets of source rocks which make contribution to hydrocarbon accumulation of the south of Lixian slope. The most important one is the "spe-cial lithologic section" in lower Es¹ (Es^1x). The "spe-cial lithologic section", which has a special lithologic component, is comprised of dark mudstone, oil shale, biological oolitic limestone and dolomite of shallow water. Its thickness is about 100–300 m. It is immature but spreading almost all over the slope, especially in inner slope (Fig. 4). But it is less developed in the south of the slope because of abundant sand supply (Yang et al., 2010a; Zhang et al., 2001).

Figure 4.  Source rock distribution of Lixian slope.

Another is dark mudstone in Es³. It is mature oil and distributed mainly in the Suning sag and the Ren-xi sag (Fig. 4). The supply range is smaller than the one in Es^1x.

These two source rocks provide hydrocarbon for different layers and areas. The "special lithologic sec-tion" normally shortly migrates and provides hydro-carbon for Es^1x–Es^3s reservoirs. But in the south, like the Gaoyang low uplift, where the "special lithologic section" less developed, hydrocarbon migrates long distantly from down dip direction of the Suning sag. Source rock in Es³ provides hydrocarbon only for Zhaohuangzhuang area because of the long distance from the source kitchen to the slope.

As discussed above, the large scale faults like Gaoyang fault, Dabaichi fault are characterized of episodic development. Since they are stagnant during hydrocarbon expulsion period (Ng-Nm), their hydro-carbon transporting ability was not likely functioned. However, the active period of small scale faults in the inner slope in the south of Lixian slope was coordi-nated with the expulsion period of "special lithologic section" in Es^1x. Those faults provided perfect migra-tion path for hydrocarbon vertical migration (Li et al., 2004; An et al., 2001).

In the view of plane migration, connected sand bodies in the south of Lixian slope were the "high-way" for hydrocarbon migration.

Since Rennan fault's activity was weak and the structural background of south Lixian slope was sim-ple, the two big fault zones (Gaoyang fault and Da-baichi fault) were the primary hydrocarbon accumula-tion areas. Currently, most of the reservoirs discovered in the south of Lixian slope are fault-nose type, close-ly related with Gaoyang fault and Dabaichi fault. The Gaoyang fault is a continuous long fault without any large interval, so is the reservoir which sealed by it. The reservoirs besides the Gaoyang fault have a good continuity, and formed a long strip play. While the Dabaichi fault is a fault zone that is composed by sev-eral small NE direction faults which are disconnected. The reservoirs attached to these faults are isolated and "bead-formed" shape. They are "one fault-one reser-voir" (Fig. 1).

The reservoirs in the south of Lixian slope are shallow buried. The thickness of each layer is thin but multi layers stacked. Except for updip pinch out litho-logic reservoirs in Es^3s near Zhaohuangzhuang area, most of the reservoirs in Lixian slope are found in Es^1x or even shallower layers whose burial depth are less than 2 400 m. The multilayer hydrocarbon bearing strata stack vertically along the faults, and presents "toothbrush" shape (Fig. 5).

Figure 5.  Reservoir distribution profile pattern in the south of Lixian slope. ① fault nose structural reser-voir; ② updip pinch out lithologic reservoir.

The property of oil generated from Es^1x shows "four high and one low" characteristic, namely high density, high viscosity, high sulfur content, high re-sin-asphaltenes and relatively low wax. The density is 0.87 to 0.93 g/cm³, 0.91 g/cm³ on average; viscosity is 27.6 to 1 607.84 mPa·s, 517 mPa·s on average; sulfur content ranges from 0.14% to 1.00% with an average of 0.63%; resin-asphaltenes is 26.4% to 59.8%, 46% on average; wax content is 3.2% to 16.9% with an average of 11.41%, and its freezing point ranges from 31 to 42 ℃, 35 ℃ on average (Liang et al., 2001). On the contrary, oil came from Es³ shows totally dif-ferent situation. It is low density, low viscosity, low sulfur content, low resin-asphaltenes and relatively low wax, which represent mature oil generated from mature source rock.

Saturated hydrocarbons chromatography distri-bution chart also tells the same story (Fig. 6). Oil in the south of Lixian slope contains high isoparaffin and cycloalkanes, especially phytane. The chromatogram shows a unique high value which is higher than any other N-alkanes. This feature is identical to the source rock of "special lithologic section" in Es^1x. But the oil chromatogram in inner slope near Zhaohuangzhuang area is significantly different from those in outer and middle slope. Its value shows relatively regular. The change of saturated hydrocarbon chromatography suggests that most of the oil from Lixian slope is came from immature source rock in Es^1x. Due to shortage of source rock in outer slope in the south of Lixian slope, the oil generated in Es^1x from middle and inner slope migrated long distantly towards outer slope through connected sandbodies, leaving heavy content of the oil remain in the lower region. Thus causing density and viscosity gradually decline from inner slope to outer slope (Fig. 7). In Zhaohuangzhuang area, oil came from Es³ near the Suning source kitchen (Zhang et al., 2009; Wang et al., 2008).

Figure 6.  Distribution of saturated hydrocarbons chromatography in the south of Lixian slope.

Figure 7.  Distribution of density and viscosity in the south of Lixian slope.

The primary migration mode is lateral migration in the south of Lixian slope. However, it is hard to summarize the pattern of hydrocarbon accumulation with single profile model. In this case, a profile model and a plane model are established to illustrate the hy-drocarbon accumulation model.

Small faults in the inner slope activated during expulsion period, which provide migration path to shallow reservoirs for the oil generated from "special lithologic section" in Es^1x. Southern Lixian slope is the primary sediments depositional area, thus sand-stone are well developed and connected in each layer. These connected sandbodies could transport hydro-carbon from inner slope to outer slope in Es^1x, Es^1s and shallower layers in "parallel transportation type" rather than "stepped-like type". Hydrocarbon was sealed and trapped by Dabaichi fault and Gaoyang fault when migrated, and formed a "toothbrush" shape fault nose structural reservoir or structural-lithologic combination reservoir (Zhang et al., 2006; Du et al., 2002). Meanwhile, when hydrocarbon migrated up-wards, suitable lithologic traps could be filled with hydrocarbon when encounted. This kind of lithologic traps maybe the next favorable plays in the south of Lixian slope.

Since a long distance from Suning source kitchen to the main parts of Lixian slope, moreover, the regional distributed effective "mud neck" cap rock in Es² made the hydrocarbon generated from Es³ source rock in Suning sag only accumulated in Es^3s near Zhaohuangzhuang area. In summery, the profile ac-cumulation model in the south of Lixian slope could be summarized as "dual source rocks generating; connected sandbodies parallel transporting; shallow fault nose traps accumulating" (Fig. 8).

Figure 8.  Profile accumulation model in the south of Lixian slope (B-B' profile as in show in Fig. 6).

Hydrocarbon parallel transported through con-nected sandbodies from inner slope to outer slope in each layer, and accumulated mainly around two large scale faults. As successive developed structure high, the axial regions of Dabaichi nose structure and Gaoyang nose structure were the main hydrocarbon accumulating and transporting structures.

Two main big fault zones in the outer slope remained stagnant during hydrocarbon expulsion pe-riod, and acted as effective fault blocks. The reservoirs distribution in the outer slope was controlled by the continuity of Dabaichi fault and Gaoyang fault, as well as the charging level. Dabaichi fault is an inter-mittent fault zone. After sufficiently charging, oil and gas would spill over through the discontinuous points and continue to migrate to even higher structure. Since Gaoyang fault located in the highest point of Lixian slope, and it extend of long distance without any break off, hydrocarbon were migrated and trapped in it. Re-servoirs were found distributing along the root of Gaoyang fault. Its shape looks like an "oil dragon" (Fig. 9).

Figure 9.  Plane accumulation model in the south of Lixian slope.

South Lixian slope is a large wide-gentle successive developed sedimentary slope because of simple structure and less developed fault. Shallow-water braided delta was the dominant sedimentary facies, characterized of "mud included within sand" and ex-cellent lateral connectivity. The activity period of small faults in the inner slope near Suning source kitchen were in accordance with the Es³ source rock expulsion period. The faults acted as vertical migra-tion path. Two large scale faults, namely Dabaichi fault and Gaoyang fault were stay still during expul-sion period and act as fault blocks.

It was believed that hydrocarbon was "stepped-like" migration model, which means oil and gas generated from Es^1x and Es³ source rock in the inner slope transporting through small faults step by step towards outer slope high positions. After fine structure analysis and faults interpretation, the accu-mulation model in south Lixian slope is considered as lateral parallel transporting. The hydrocarbon mi-grated vertically to the shallow layers through small faults near Suning sag, then parallel transported to-wards high positions by lateral connected sandbodies. The "L" type migration system was established by the faults and sandstone. Migrated hydrocarbon then trapped by Dabaichi fault and Gaoyang fault in the outer slope.

Currently, the exploration level in the south of Lixian slope is relatively low. This new hydrocarbon accumulation pattern could be used to instruct oil and gas exploration in the following aspects.

(1) The main source rock in Es^1x distributed al-most all around Lixian slope, and the middle slope was the only pathway which hydrocarbon must be transported. Therefore, subtle traps in the middle slope have as bright future for petroleum discovery as any other. More research should be done about fine seis-mic interpretation and microfacies characterization in middle slope to look for small structure high and li-thologic geobodies.

(2) Shallow layers such as Ed, Ng, Nm, etc. were also connected by small faults. It is reasonable to take those shallow layers as important exploration strata; especially in the SW direction of the slope for that SW is a regional structural high.

Grateful thanks should be given to the editors who devoted much time in correcting this manuscript.