Storm gravity flow sedimentary system

The central uplift belt of Huimin sag is located in the central and western part of Jiyang depression, extending from east to west, which divides Huimin sag into Zizhen sag in the north, Nanlin sag in the south and Lingxian uplift in the west. The early central uplift zone of the third member of Shahejie Formation has not yet formed, and this area is a semi-deep lake-deep lake deposit; In the middle period of sedimentation, with the activity of Linyi fault, the uplift prototype was gradually formed. With the increase of the activity intensity of Linyi fault, the uplift scale expanded from west to east, and the water body in the lake basin deepened. River delta deposits are developed in Tangzhuang and Linyi areas in the west of the central uplift belt, and semi-deep lake-deep lake mudstone and oil shale deposits are developed in the east of Linyi. In the late third member of Shahejie Formation, the intensity of tectonic movement weakened, and the lake basin began to shrink and gradually stabilized. The lake basin is shallow, with river delta, beach bar and storm gravity flow deposits. The following will focus on the sedimentary characteristics and sedimentary models of rainstorm flow deposition.

I. Sedimentary characteristics

1. Petrological characteristics

The lithology related to storm deposition in the upper part of the third member of Shahejie Formation in this area is mainly fine sandstone, silty sandstone, mudstone, biolimestone and sandstone containing bioclastic. Part of the fine sandstone contains fine mudstone, and the gravel diameter is 0.5 ~ 3.0 cm. The biological debris is mainly composed of relatively complete snails and ostracods. According to microscopic statistics, the content of debris in siltstone is high, with an average of 25% and a maximum of 35%. The composition is terrigenous polycrystalline quartz, flint, intermediate-acid extrusive rock and mudstone cuttings. The content of heterogroup is low, generally less than 65438 05%.

2. Sedimentary structural characteristics

Through core observation, the sedimentary structures reflecting storm flow deposition are very rich and diverse (Figure 8- 19):

(1) Quasi-syngenetic erosion structures When strong water flows over the surface of sediments, various erosion structures are formed. The common erosion structures in this area are scouring surface and cutting structure. The unevenness of scouring surface reflects the magnitude of storm action, and the scouring surface in this area is gentle wave-like and groove-like (Figure 8- 19A, b, c). The cutting structure diagram (Figure 8- 19D, e, f, g) is one of the structures that cause the top surface of sandstone in this area to be uneven. Rutian 5-5 (Figure 8- 19D) shows that one side of parallel bedding sandstone is higher, the other side is flattened, and the cutting angle is larger, which is filled with argillaceous sandstone, which is caused by the storm. After the storm, the lake returned to calm, so the mudstone in the deeper lake (under clear waves) covered the shear surface, which is evidence that the storm flow affected the lake bottom.

(2) Quasi-contemporaneous deformation structure Quasi-contemporaneous structures are developed in this area (Figure 8- 19H, I), mainly including heavy-load structures, spherical pillow structures, flame structures and underwater dikes. For example, in Well Tian 5-5, the diameter of the spherical pillow structure sandstone pillow is about 2cm, and it has stripes, but it has been deformed, and the deformed stripes bend downward into a groove. The underlying mudstone extends into the sand layer in a tongue shape. Generally speaking, quasi-contemporaneous deformation structures rarely appear in traction current, but mostly appear in gravity flow deposits. Sand and mud are mixed, and the accumulation speed is fast, so it is too late to drain water, thus forming excess pore water pressure, and then forming a series of deformation structures. Rainstorm flow deposition has the characteristics of gravity flow, so its sediments have the characteristics of gravity flow deposition.

(3) The tempestite in the upper third member of Shahejie Formation in the central uplift zone of Huimin sag has various bedding types and characteristics. There are mainly gradient bedding, parallel bedding, mountain cross bedding, wavy sand bedding and lenticular bedding.

Graded bedding is located on the scouring surface, generally 10 ~ 20 cm. The graded bedding seen in the core is positive grain order, without reverse grain order (Figure 8- 19J), and it is in abrupt contact with the underlying mudstone. It is formed by the rapid settlement of storm density flow according to gravity difference and the weakening of eddy current support after the peak of storm. The grain sequence layer represents that the wind is weakened and the gravity is greater than the shear force.

Parallel bedding Parallel bedding (Figure 8- 19K) is found in medium-fine sandstone. Stripping lineation structure exists on the sandstone stripping surface (Figure 8- 19L). Stripping lineation generally produces a high leveling layer, which represents the high-energy environment of storm flow activity.

Hill-shaped and concave cross bedding Hill-shaped cross bedding is found in storm rocks in Linyi, Tianjia and Shanghe areas, with clear internal structure and a height of 1 ~ 3 cm. There are three types of hilly and concave cross bedding in this area. Answer: The upper fine layer of this series is dome-shaped, and the lower fine layer is parallel to the lower interface (Figure 8- 19M). B: The fine layer of the layer system is convex on the top and concave on the bottom, and the lower grain layer at both ends intersects with the upper grain layer (Figure 8- 19O). C: The lower fine layer of the series is concave, and the upper fine layer is parallel to the upper interface (Figure 8- 19N).

Hilly cross bedding in this area appears in storm rocks in front of the beach, and is located on parallel bedding or massive bedding. From the position point of view, it is not the product of the peak period of storm action. Most researchers believe that it is formed by weak oscillating water flow and multi-directional water flow when storm waves weaken, and it is a sign of storm action.

Figure 8- 19 reflects the sedimentary structure characteristics of storm wave action.

Ripple is not only a cutting structure, but also one of the structures that cause the top surface of storm sandstone to be uneven. The wave marks seen in this area are straight ridges or tuning forks, slightly symmetrical, with a wavelength of about 3-5 cm and a wave height of 0.3-0.6 cm (Figure 8- 19P). Dendritic biological boreholes are developed on the surface of tuning fork-shaped wave marks, which is the most common distinguishing sign of wave activity. The ideal storm sequence and turbidity proposed by T.Aigner( 1982).

In addition, the biogenic structure also includes biological crawling trajectory, biological hiding hole and biological escape trajectory. The trace of biological escape in this area is generally located in the lower part of the storm sequence, which is a slender vertical hidden cave. Backfill structure is not developed, which represents the occurrence of event deposition. When rapid deposition occurs, organisms escape upward in order not to be buried. This is because after the storm wave weakened, the bottom layer under the clear wave resumed to be temporarily oxygenated, so the organisms began to move and the benthos began to flourish.

3. Particle size characteristics

In this area, it is considered that the particle size probability curve of storm-induced sand layer is two-stage and multi-stage The two curves consist of jumping population and floating population (Figure 8-20). The content of jumping population is about 60%, the φ value of S tangent point is 3.0 ~ 3.5, and the dip angle of jumping population is about 70. Multi-segment curve jump is generally composed of 2 ~ 3 segments, which reflects that there are many boundary points of thrust back, and the overall dip angle of jump is greater than 60. These two curves have high slopes and are well selected, showing the sedimentary characteristics of storm gravity flow.

Figure 8-20 Probability curve of rainstorm deposition particle size

4. Vertical sequence

The change of storm hydrodynamic conditions formed the corresponding sedimentary sequence and sedimentary characteristics of each stage. The sedimentary sequence of storm flow in this area was deposited under the condition of alternating normal weather and storm weather in shallow lakes. According to a large number of core observations, an ideal vertical sequence-"Ma Bao-like sequence" is abstracted.

(1)Sa graded bedding section, thickness 10 ~ 20 cm, with obvious scouring surface at the bottom, which contains fine mud and gravel.

(2)Sb is a parallel bedding section or a massive bedding sandstone section with a thickness of 15 ~ 25 cm, and the lithology is fine sandstone-medium sandstone, with traces of biological escape.

(3) (3) Silt-sandstone profile of SC hill-shaped cross bedding and wavy sand bedding is about 20cm thick, which is a unique profile in storm flow deposition.

(4)Sd intermittent streaks, lenticular layered silty sand, argillaceous silty sand and silty mudstone profiles, with a thickness of 20 ~ 30cm, flat top and bottom, and traces of foraging inside, are formed by suspended solids in low flow state in the later stage of the storm.

(5) The profile of mudstone and shale in normal 5)Se weather is about 30cm, which represents the fine-grained deposition of normal shallow lake-semi-deep lake.

The above sedimentary sequence reflects the energy attenuation process of storm density current and storm wave.

Two. Types of storm deposits

Due to the difference of geographical location and water depth, the sedimentary sequence and characteristics of storm rocks in different regions are different. According to this, two types of storm rocks can be divided, which represent the sediments near the storm wave datum and above the normal wave datum respectively. The stratum model is shown in Figure 8-2 1.

Figure 8-2 1 Storm Gravity Flow Deposition Model of Upper Shahejie Formation

1.Ⅰ tempestite

The (1) delta and remote storm rocks in front of the beach are formed near the storm wave bottom, represented by well Shang 13- 107. The sedimentary sequence consists of Sbc, Sce and Sde, and the sediments are obviously fine, mainly silty sand and argillaceous silty sand, often mixed with gray-black mudstone, with horizontal bedding and massive bedding. Wave bedding, horizontal bedding and lenticular bedding are common in storm rocks, and wave marks are often displayed on the peeling surface of sandstone.

(2) Near-source storm rocks in delta and beach are located in the middle and upper part between normal wave bottom and storm wave bottom, represented by Lin 10- 1 well. The sediments are siltstone and gravel sandstone, and waves, grooves and mound erosion surfaces are common at the bottom. Gradual, massive, parallel, mound-like cross bedding and wave-crossing cross bedding are developed inside, and wide and gentle wave marks are common on the top surface of sandstone. Storm rocks near the upper beach of the third member of Shahejie Formation in this area are well developed, with common sequences of Sa, Sabc and Sac.

2. Type 2.II storm rocks (shallow water storm rocks behind the beach)

Storm flow is located between the beach bar or estuary bar and the coast, carrying sediments through the main body of the beach bar or estuary bar, reducing energy. At the same time, due to their blocking effect, the sediments carried by storm flow are deposited between beach bar or estuary bar and coast. Characteristics of type ⅱ storm rocks: ① sand and mud are mixed, with poor sorting, containing a large number of plant debris and a large number of biological fossil fragments; ② Mountain-shaped and sag-shaped cross bedding is undeveloped, mostly massive or gradual bedding. During the storm, this area suffered erosion and erosion, and many erosion surfaces developed. ③ It is often covered by carbonaceous shale or sandwiched between carbonaceous mudstone. Type Ⅱ storm rocks form the lakeside area above the bottom of Apollo, and the sediments carried by storm waves are deposited near the earthquake source, mainly in the west of Linshen 1 Well.

From type I storm rock to type II storm rock, the water depth becomes shallow, the storm action is enhanced, the erosion surface fluctuates, the grain size value increases, and the percentage content of sandstone increases.

Third, the plane phase mode

In the late period of the third member of Shahejie Formation, the terrain in this area became flat, the water body became shallow, and delta and beach bar deposits developed. The provenance comes from Lingxian uplift in the west and Chengning uplift in the north. Due to the transformation of storm waves, storm rocks are formed in Linyi, Tianjia and Shanghe (Figure 8-22). Type I storm rocks are distributed in well Lin 10- 1- tian5-5 and Shanghe area in the east of the lake basin, and are transformed from beach bars and delta front sand in shallow lakes along the northwest coast. Class Ⅱ storm rocks are located in shallow water area west of Linshen 1 well and swamp area north of beach bar.

Figure 8-22 Sedimentary facies plan of the upper member of Shahejie Formation

Fourthly, the regularity of storm deposition.

(1) The storm deposits in the west of the central uplift belt in Huimin sag are mainly developed in the upper part of the third member of Shahejie Formation, and occasionally in the upper part of the fourth member of Shahejie Formation. At the end of the fourth and third member of Shahejie Formation, the lake surface is vast, the terrain is not steep, and the marginal debris is rich, which is easy to produce storm backflow materials and form storm deposits.

(2) On the stratigraphic section, storm deposits and non-storm deposits appear alternately, with the former accounting for only a small proportion. According to the statistics on the thickness of storm deposits and non-storm deposits in coring intervals of Shang 25-24, Shang 13- 107, Lin 1 0-,Tian 5-5 and1,the following table is obtained:

Study on Sequence Stratigraphy and Sedimentology of Sand Bodies —— A Case Study of Huimin Depression, Shandong Province

(3) On the plane, storm rocks, especially Type I, are surrounded by non-storm rocks, with deltas and beaches in the shallow land direction and turbidite or semi-deep lake or deep lake mud shale in the basin direction. The thickness of storm rock near the source area is large, the sand-mud ratio is high, and the sand layer is thick, which decreases towards the far source area.

(4) Compared with turbidite, tempestite is fine in particle size (mainly silty sandstone), good in sorting, rounded and low in matrix content, which is beneficial to the development of reservoir space and the seepage of oil and gas. The tempestite core of Linshen 1 well contains oil, which can constitute a kind of reservoir, adding a new type to the lacustrine reservoir in the west of the central uplift belt of Huimin sag.