Division and Characteristics of Shale Parasequences in the Upper Fourth Member of the Shahejie Formation, Dongying Depression, Bohai Bay Basin, China

Shale parasequence analysis is an important part of sequence stratigraphy sudies. This paper proposed a systematic research method for analyzing shale parasequences including their delineation, division, characteristics and origins. The division method is established on the basis of lithofacies. Multi-method analysis and mutual verification were implemented by using auxiliary indicators（such as mineral compositions, geochemical indicators and wavelet values）. A typical shale parasequence comprises a lower interval of deepening water-depth and an upper interval of shallowing water-depth（e.g., a shale parasequence including a high-total organic carbon（TOC） shale-low-TOC limy shale）. Abrupt increases in pyrite content, TOC value, relative hydrocarbon generation potential（（S1＋S2）/TOC）, and wavelet values are indicative of parasequence boundaries. The proposed research method was applied to study the upper fourth member of the Shahejie Formation in the Dongying depression, Bohai Bay Basin. Results show that there were seven types of parasequences developed. A singular and a dual structured parasequences were identified. Three factors controlling the development of the shale parasequences were identified including relative lake level change, terrestrial input and transgression. The development of high-TOC（〉2%） shale parasequences was mainly controlled by biological and chemical sedimentation. The low-TOC（〈2%） shale parasequences were mainly deposited by chemical sedimentation. The diversities of shale parasequences were caused by four major controlling factors including climate, relative lake level change, terrestrial input and emergency（e.g., transgression）.

Correlation of Upper Devonian and Lower Carboniferous Depositional Sequences in Different Facies Belts on Southern Margin of Upper Yangtze Platform

A fundamental problem in sequence stratigraphy is whether the sequences are global in extent and whether the sea level changes are eustatic in nature. Study on the Upper Devonian and Lower Carboniferous depositional sequences in different facies belts on the southern margin of the Upper Yangtze platform shows that different orders of sequences possess distinctive distribution and correlation in space, although the sea level changes in various orders of sequences may be primarily global in nature. The present paper regards the distribution in space, especially in different facies belts, as a criterion for defining and recognizing the various orders of sequences. The orthosequence (third order sequence) is probably global in nature, which may be discerned in various depositional facies belts on the continental margin and can be correlated over long distances, sometimes maybe worldwide. Commonly, correlation of the subsequences (fourth order sequences with time interval of 0.5 Ma to 1.5 Ma) is difficult in different facies belts, although some of them are probably also worldwide in distribution. A subsequence should be distinguishable and correlatable within at least one facies belt. The higher order sequences, including parasequences (fifth order sequences) and microsequences (sixth order sequences), are regional or local in distribution. They may have reflected the longer and shorter Milankovitch cycles respectively. Orthosequences and subsequences are usually recognizable in different facies belts, while parasequences and microsequences may be distinguished only in shallow marine deposits, but not in slope and basin facies deposits.

Sequence Stratigraphy of the Palaeogene in the HGZ Area of Qaidam Basin

Sequence stratigraphy can be used to predict the oil and gas reservoir bodies and to choose the oil targets. Fan delta and sublacustrine fan systems are developed in the HGZ Area, which is located in front of the Altun Mountain in the west of the Qaidam Basin. On the basis of seismic and well drilling data, the deposits in the area were studied by using sequence stratigraphy and reservoir prediction techniques. Various reservoir prediction techniques used under the constraint of high resolution sequence framework could help to improve the precision of reservoir prediction and to recognize the pinch out line of sand layers, the distribution of isolated sandstone bodies and the types of oil pools. The study of sequence stratigraphy makes reservoir prediction more effective by dividing different scales of sequences and distinguishing the system tracts and parasequence sets, and indicates the internal relationship between oil/gas and sedimentary bodies in different system tracts with evolution in time and space.

Sequence Stratigraphy and Relative Sea Level Changes in Sinian System of Yichang, Hubei Province

The study on outcrop sequence stratigraphy of the Sinian section in the Yangtze Gorges an important step in the comprehensive stratigraphy for the Sinian stratotype. In this paper, the base of the Nantuo tillite is selected as the bottom boundary of the Sinian, and the top of the Baimatuo Member of the Dengying Formation is taken as the top boundary. The Sinian lasted from 740 the to 570 Ma. According to the features of sequence boundary facies and the parasequence sets, two supersequences are recognized in the Sinian with six sequences. In this paper, the authors discuss the correlation of those sequences. On the basis of the subsidence rate and the thickness of decompressed deposits, the authors discuss the mechanism of tectonic and sedimentation of the Dengying Formation and the features of high-frequency rhythms as well as the 3rd-order sequences in the Yangtze Gorges.

Outcrop Sequence Stratigraphy of Neoproterozoic in Hunjiang Area, Jilin Province

Three depositional sequences have been recognized in the Neoproterozoic of southern Jilin. In this paper, we focus discussion on the parasequences and their stacking patterns in Wanlong, Badaojiang and Qinggouzi formations and on their diachronism by the theory of sequence stratigraphy, event stratigraphy and chronostratigraphy. The authors advance a new comparison of the Neoproterozoic between Jilin and Liaoning provinces.

Using Gamma Emissions to Identify Cycles in the Lower Cambrian Monkton Formation (NW Vermont): Implications for Identifying Sea Level Variation on the Iapetus Margin

The Monkton Formation is described as a Lower Cambrian regressive sandstone unit containing shallowing-up cycles, called parasequences, which record tidal flat progradation. Spatial variation and limited outcrops of continuous stratigraphy have made it difficult to characterize how cycles change in architecture and thickness through the entirety of the Monkton Formation. This study seeks to identify stratigraphic trends in the parasequence architecture and thickness from the successful recognition of facies in the subsurface, information that will clarify how the sea level changes and impacts accommodation space. New geophysical data allow this research project to explore the stratigraphy of the Monkton at higher levels of resolution than previously achieved. Initial statistical study of gamma ray data from a well through the Monkton suggests that meter-scale parasequences are identifiable in geophysical logs. Using continuous gamma log data, this work identified clastic carbonate ratios from a 1034’ deep geothermal well drilled at Champlain College in Burlington, VT. Although cycles were identified in the gamma log, the spatial variation, complexity and variety of parasequence types within the Monkton made it difficult to correlate the sequences with specific environments or changes in sea level. To see if specific architectural elements of parasequences and lithologies could be characterized by gamma values, outcrops of the Monkton were surveyed containing both parasequences and lithologies that were useful paleobathymetric indicators. The survey determined gamma ray patterns and relative values of a common Monkton parasequence representing tidal flat progradation and a carbonate lithology indicating an abrupt sea level rise. Gamma data from a second well on the University of Vermont campus, along with accompanying borehole camera video, confirmed that the outcrop survey results could be useful in subsurface interpretations of the Monkton. Intervals representing the parasequences and carbonate lithologies were identified in the Champlain College Well and were combined with the UVM Fleming Well findings to create a composite stratigraphic section of the Monkton. The parasequences and lithologic trends identified in the composite section indicated that, over the entirety of the Monkton, accommodation space decreased. The decrease in accommodation space is interpreted to represent a change in the rate of sea level rise. The Monkton Formation deposition occurred in a transgressive systems tract with varying rates of sea level rise, which implied that the overlying Winooski Formation would represent highstand systems tract deposition.