Sequence stratigraphic analysis of the first layer, upper second submember, Shahejie formation in Pucheng oilfield

INTRODUCTION

Sediment of the first layer, upper second submember, Shahejie Formation from Pucheng oilfield in which sedimentary facies were terminal fan, was once recognized to be a simple lacustring transgression process.However, accompanying the development of the high-resolution sequence stratigraphy, the older point needs to be changed.High-resolution sequence stratigraphy is based on the identification of the smallest stratigraphy units (Van Wagoner et al., 1990) or genetic stratigraphic units (Cross and Lessenger, 1998; Galloway, 1989, Vail, 1977), which reflected relative sea-level variations in marine environments or base-level variations in continental deposits.Terminal fan is characterized by a progressive downstream reduction in discharge due to infiltration and evaporation processes, such that no water leaves the system as surface flow (Kelly and Olsen, 1993).Therefore, it is such a significant thing that this article researches the target area defined as terminal fan by applying high-resolution sequence stratigraphy.

GEOLOGICAL SETTING

Dongpu depression is a typical basin in eastern China.It is a subset unit of the Linqing sag in the Bohai Bay basin, stretching in the NNE direction.The area is 5 300 km². The tectonic evolution of the basin includes an Early Paleozoic–Triassic cratonic basin cycle and a Cenozoic rift basin cycle.

Pucheng oilfield is located in the northeast of Dongpu depression (Fig. 1).Pucheng structure is an inherited structure uplifted from low-lying land, and the conformation of which is a long-axis anticline complicated by faults, which is 15 km in south-north orientation and 4.5 km in east-west orientation.

Figure  1.  Location of the target area.

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The first layer, upper second submember, Shahejie Formation is the targeted area (Fig. 2) and substituted by "S₂s₁" for the reason of convenience in this article.

Figure  2.  Chart of tectonic strata of Cenozoic Formation in Pucheng zone (target layers are signified by bold frame).Fm.Formation; Mem.member.

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FACIES ANALYSIS

A simple model is presented for sand-dominated and mixed-load terminal fan. The model utilizes a subdivision of system into feeder, distributary, and basinal zones (Kelly and Olsen, 1993). This is largely not only based on descriptions of terminal fan systems but also incorporates the aspects of a closely related system.

Feeder Zone

The feeder zone is dominated by the main feeder channel and associated interchannel areas.The sediment bodies are composed of relatively coarse sandstones or conglomerates (although finer-grained sandstones may be expected in the absence of coarse debritus) (Kelly and Olsen, 1993).However, no distinctive feeder zone has been recognized within the target area (S₂s₁).

Distributary Zone

The sandstone bodies are interpreted as the products of low-sinuosity streams forming the distributary channel of the terminal fan system (Graham and Reily, 1972). In a distributary zone, character of channel, proximal overbank, and distal overbank can be identified according to the core data.

The channels were probably formed by relatively small high-energy streams that often carried dune fields (Zhang et al., 2007).Distributary channel sandstone bodies are mainly medium-grained sandstones with minor siltstones and claystones.Channel may reach 7 m in thickness and can be subdivided into a series of stacked fining-upward subunits of roughly 3m thick.A transition surface is defined as an interface cross, where lithofacies changes, indicating changes in hydrodynamic conditions.Scoured bases marked by mud-chip lags up to 0.5 m thick are common.Finingupward units show an upward transition from trough cross bedding (Fig. 3a) to parallel stratification (Fig. 3b) and ripple-laminated beds (Fig. 3c).Channel erosion is indicated by the scoured bases and the presence of a basal lag (Fig. 3d).

Figure  3.  Terminal fan sedimentary structure of the S₂s₁. (a) Trough cross bedding; (b) parallel bedding; (c) ripple-laminated beds; (d) mud rock on the channel basal lag; (e) sand-shale alternating layer; (f) bioturba-tion structure.

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Within proximal and distal overbank of the distributary zone interbedded by lower medium-to finegrained sandstone, cross-stratified fine-grained sandstones, very fine grained asymmetric-ripple laminated sandstone, and siltstone and claystone are recognized (Fig. 3e). These units can be attributed mainly to sheetflood process resulting from channel flooding.The distal overbank is considered transitional to the basinal zone.

Basinal zone

The basinal zone of terminal fan is probably governed by the suspended bedload ratios of the terminal fan systems (Kelly and Olsen, 1993). It will generally only receive very fine-grained sediment after large floods, and distributary channels will extend into this zone only during extreme flood events. The basinal zone contains mudstones and siltstones with very minor sandstones.

First layer, upper second submember, Shahejie Formation from Pucheng oilfield (S₂s₁) is interpreted as the deposits of a semiarid terminal fan system that was marked by a systematic downstream variation in channel morphology (Zhang et al., 2007).

The simple model presented below (Figs. 3 and 4) summarizes the facies and architecture of terminal fan deposits.

Figure  4.  Facies model for terminal fans. (a) Plan of sedimentary model; (b) profile of sedimentary model.

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In the feeder zone, discrete low-sinuosity feeder channels transported abundant sandy and gravelly bedload across a generally finer-grained and low-relief alluvial plain.These passed downstream into a distributary zone dominated by mobile low-sinuosity channels that deposited multistory sand bodies. Fine sands and silts accumulated from low-energy residua flood discharge. Poor preservation of fine sands and silts reflects channel mobility within this zone. Further downstream, this system passed gradationally into the basinal zone, where phases if high-energy flooding punctuated a more quiescent regime characterized by episodic low-energy shallow sheet flooding and protracted periods of subaerial exposure and nondeposition. The high-energy floods appear to have been centered on transient and low-sinuosity ephemeral channels.

Well-Log Facies Characterization

Well-log analysis is used to calibrate electrofacies with sedimentary facies and to infer electrofacies associations and depositional environments directly from well-logs (Bourquin et al., 2006, 1998). It is essential to identify and correlate sedimentary sequences, especially those of continental ones in view of highresolution recognition.

Most electrical methods log the resistivity of the rock around the borehole.The specific resistivity shows what type of rock it may be in general, how many conducting parts, and how many nonconducting parts like lime, silica, and so on, it contains. Oil and gas within the pore space of a sand or sandstone will increase its resistivity because they are nonconductive materials. It will show lower conductivity or alternatively higher resistivity on the diagram.

The so-called conventional resistivity logs show a strict interdependence of their depth of penetration and their resolution of petrographic details. If a deep penetration is needed to log the pay out of the zone of mud filtration, the resolution is very bad, and vice versa. Better results are shown by the microspherically focused methods where the logging current is compressed to a disk and is forced to penetrate relatively deep into the rock at a good rate of resolution.

Nuclear methods have a great advantage of being nearly independent of the type of mud.The most usua type is the gamma ray log. The probe usually contains a scintillometer, and the diagram shows the gamma ray of the rock recorded against the depth (Hong 1998).

As the result of the study of classical well-logs (gamma ray, microspherically focused log) calibrated by cores, four electrofacies are identified as characterizing the main sedimentary facies of the S₂s₁. These four electrofacies have been validated from a study of core and well-logs based on 900 wells.With classical well-logs, one of the electrofacies may correspond to an association of sedimentary facies (see Table 1 and Fig. 5).The logs of microspherically focused resistivity and gamma ray in distributary channel (Ⅰ) reveals bell-shaped curve indicating typical upward-fining sequence and box-shaped curve signifying massive channel sand body.Mudstones in basinal zone where electrofacies belong to lake or floodplain (Ⅳ) are abundant in radioactive minerals producing high gamma-ray and low microspherically focused resistivity values.Two bottoms in gamma-ray log are recognized and used as the boundary of S₂s₁ (Fig. 6).

Table  1.  Four electrofacies characterizing the main sedimentary structure of the S₂s₁ from well-log data

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Figure  5.  Well-log characterizing the main sendimentary facies based on real cores and log data.

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Figure  6.  Well-log and core analysis showing sedi-mentary environment variations and genetic se-quences (well P1-16).

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HIGH-RESOLUTION SEQUENCE STRATI-GRAPHY ON CORES AND WELL-LOG ANDVERTICAL STACKING PATTERN

Sequence Analysis

The first layer, upper second submember, Shahejie Formation, exhibits three orders of stratigraphic cycles: (1) genetic sequences, (2) genetic sequence sets, and (3) minor stratigraphic cycles.The base-leve variations determine the sedimentary facies of one genetic sequence (Fig. 6). A genetic sequence is usually represented by a series of erosion and transit periods (amalgamated channels that may represent periods of base-level fall) and aggradation periods (baselevel rise) (Cross and Lessenger, 1998). Periods of erosion, transit by-pass correspond to a low facies preservation and periods of aggradation corresponding to a high facies preservation.

Periods of high preservation, i.e., base-level rise, correspond to (1) channel-flood to channel-flood plain or channel lacustrine sequences, ranging from 3–7 m thick, wherein channel deposits are thinner and flood deposits or flood plain, and lacustrine deposits are thicker, and (2) grain size becoming finer.

Periods of low facies preservation, i.e., base-level fall, correspond to erosion periods with no preservation of lag.

In a terminal fan system, the base-level rise is characterized by high facies preservation: the vertical development of several sequences of distributary channel systems, overflow deposits, and flood plain deposits. During the base-level fall, the distal flood plain deposits grade vertically into flood deposits and periods of erosion or transit by-pass.

By observing the stacking arrangement of genetic sequences, five genetic sequence sets and one minor stratigraphic cycle can be identified in the targeted area (Fig. 6). Containing two symmetrical base-level half cycles from rise to fall, genetic sequence set is deposited in high accommodation for receiving the sediment supply. The axis of symmetry marks the flooding surface dividing the sequence into a rise half cycle and a fall half cycle. A minor stratigraphic cycle experiences regional lacustrine transgressionregression action and source recharge.

The turnaround episode (maximum flooding period) between high-preservation and low-preservation facies is marked by well-developed clay in a flood plain environment (Cross and Lessenger, 1998). The identification of the bioturbation within lacustrine deposits characterizes the maximum flooding episode (Fig. 3f). Well-log clearly shows the transition from channel-flood deposit sequences to well-developed clay deposits that may indicate maximum flooding episodes.The value of gamma-ray log (GR) becomes high and micro-spherically focused log (MSFL) gets low, respectively.

Vertical Stacking Pattern of Genetic Sequences

The maximum flooding episodes of each genetic sequence are indicated on vertical well-logs, and the evolution of several genetic sequences can be seen Genetic sequence stacking patterns can be used to define genetic sequence sets showing base level rise and fall (Bourquin et al., 1995).

The stacking pattern is defined based on vertica log response.Gamma-ray value is relative to the contents of shale and median-sized grain, the decrease of which means a minish of shale content and an accretion of median grain diameter and corresponds to the deepening of water depth and base-level rise with a prograding stacking pattern formed.Accordingly, the opposite side is the shallowing of water depth, referring to a base-level fall and a retrograding stacking pattern (Deng et al., 2000).

Correlations

It is necessary to have closely spaced wells tha from a dense network to correlate deposits within a continental environment.This study has been integrated into a stratigraphic study of the first layer, upper second submember, Shahejie Formation (S₂s₁).

High-resolution sequence correlation is done between contemporaneous strata and interfaces but rock types. The turnaround surface during base-level fluctuation is the boundary of bipartite time unit and prior surface for correlation.Combining lithology with electrical data, four high-resolution stratigraphic frameworks are established including minor cycle and genetic sequence sets as the stratigraphic units.Three W-E and one N-S correlations of the S₂s₁ stratigraphic cycles are picked up to show the diachronous nature of the formation (Fig. 7). Applying high-resolution sequence stratigraphy, the sediment of S₂s₁ is recog nized as a result of lacustring transgression-regression process and not just a simple transgression process.

Figure  7.  W-E ① (a), W-E ② (b), W-E ③ (c) and N-S (d) correlations of the S₂s₁.

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CONCLUSIONS

Facies analysis indicates that the S₂s₁ was a ter minal fan system. High-resolution sequence stratigra phy of continental deposits is based on analysis o high-frequency fluctuations in base level as identified from sedimentological studies and calibrated on well log signatures. The correlation of the S₂s₁ allows us to characterize the stratigraphic evolution of the termina fan sedimentation.