Sequence stratigraphy is a very powerful tool for analysis of facies successions. The facies successions are formed by the interaction of tectonic, eustatic, sedimentary and climatic processes. In this paper, we present the examples of the interaction using by our studies in Japanese Islands. Namely, we firstly introduce the depositional sequence model of “EXXON Group”, secondary discuss the factors of eustacy, sediment supply and subsidence, then give the new topics of effect on stratigraphy, and finally describe the development in future and problems of sequence stratigraphy.
Sequence stratigraphy has been rapidly spreading in the world and causes various sort of debates because sequence stratigraphic interpretations related to mechanisms of deposition make possible three-dimensional estimation of facies distribution and worldwide correlation of strata. The “Exxon Model”, as a basin fill model, has been paid much attention because it displays the significance in the basic concept of sequence stratigraphy and possibilities of applications. The “Exxon Model” which has been constructed as a depositional model for passive margin basins represented by gentle subsidence and relatively stable deposition, however, might not always be applicable to some sedimentary basins without careful considerations for their tectonic and depositional settings. In the Niigata sedimentary basin, as a representative back-arc basin located on central Japan, hydrocarbon exploration for several decades has resulted in a large accumulation of subsurface geologic information. Three-dimensional studies for sequence stratigraphic framework within a back-arc tectonic setting, therefore, are available in this basin. As a result of this study, 12 third and fourth order depositional sequences, which are interpreted to have been formed by relative change of sea level, are now recognized. These depositional sequences are superimposed on a part of a second order depositional sequence considered as a tectonically controlled sequence. The features of fifth or sixth order depositional sequences are superimposed locally in the deposits of the Niigata sedimentary basin. The depositional history of the Niigata sedimentary basin can be divided into following 5 stages, based on the disclosed sequence stratigraphic framework of this basin. i) stage of sedimentary basin formation ii) stage of turbidite deposition at basin floor iii) stage of slope progradation iv) stage of shelf extension v) stage of delta progradation
A sequence stratigraphic study in conjunction with a quantitative basin subsidence analysis was undertaken in the northern part of Ishikari plain of Hokkaido to clarify the depositional history, recognize the tectonic influences and predict the sandstone reservoir distribution. An interpretation based on seismic data, well logs and biostratigraphic data indicates that the Miocene strata in the basin are composed of eight 3rd order depositional sequences which can be further subdivided into component systems tracts. In the Miocene depositional history of the basin, three stages of sedimentation are recognized by variations in the development of systems tracts and basin architecture. A quantitative basin subsidence analysis from well data indicates that each sequence boundary was resulted from a relative sea level fall and the stages of sedimentation were strongly influenced by the rate of total basin subsidence. Tectonic activity influenced the development of sequences and their component systems tracts by masking the sea level fall with periods of rapid subsidence, by exaggerating relative sea level fall and by shifts in sediment source direction as well as sediment accumulation rate as a result of regional uplift. This study indicates potentials of new stratigraphic types of plays in the basin by predicting sandstone reservoirs basinward, although the previous exploration activities have been focused on anticline-structural traps.
The Shiiya Age successions (6.0Ma to 3.5Ma), northren part of the Niigata oil fields are mainly consisted of radial-type submarine fan deposits. Using many seismic profiles and wells, we could detect five high sea-level events during the Shiiya Age, based on analysis of foraminiferal assemblages, and make clear that the deposition of the submarine fans are controlled by eustatic sea-level changes, under a stable tectonic environment of the back-arc basin of Japan Arc through the time. The eustatic controls during the Shiiya Age are as follows: 1) the sand lobes of submarine fan mainly develop in a basin center at low sea-levels. 2) each sand lobe retreated gradually from a basin center to the margin with sea-level rise. 3) the successions of the submarine fan change upward from proximal to distal facies. Those three things are fit well to the basic concepts of Mutti's (1985) model.
Sedimentary rhythm analysis is a method to study sedimentary cycles through the systematic sampling and analysis of the sequences, derivation of multiple paleoenviron-mental parameters from the analytical data, and examination of the frequencies in variation of these parameters and their phase relationships. Through sedimentary rhythm analyses, it is possible to study the dynamics of paleo-ocean circulation. In this short paper, I would like to show the application of this method to reconstruct dynamic behaviour of the surface and deep water circulation within the Japan Sea corresponding to the glacial-interglacial sea-level oscillations during late. Quaternary. Reconstruction of this sort is useful to understand the paleoenvironment as well as mechanism(s) of the petroleum source rock deposition within marginal seas.
Sequence stratigraphic interpretation was carried out by integrating biostratigr aphic analysis, well log, and seismic interpretations in the offshore Louisiana of the Gulf of Mexico. In biostratigraphic analysis, abundance of benthonic foraminifera, planktonic foraminifera and calcareous nannoplankton were investigated in the depth range from 2, 500ft to 6, 500ft of a well located in the south addition of South Marsh Islands area. Four 4th order sequences were identified through recognizing condensed sections characterized by peaks of abundance curves. In well log interpretation, high gamma-ray and low resistivity were the indicators of condensed sections. Upward coarsening and upward fining patterns were interpreted to identify sequences and systems tracts. In seismic interpretation, reflection character was used to identify sequences and systems tracts because reflection terminations such as onlaps, offlaps, downlaps were not obvious in this area. Condensed sections were characterized by continuous high amplitude reflectors. Slope fan systems tracts were characterized by hummocky reflection configurations. By integrating biostratigraphic analysis, well log, and seismic interpretations, sequence and systems tracts can be identified with great precision and confidence. This can be achieved by none of three techniques by itself.
Thirteen boreholes were drilled on and around Irabu Island in the southern Ryukyus, from which limestones of the Pleistocene Ryukyu Group (50-110m) were recovered. The seven lithologies in the Group fall into the coral limestone facies and the rhodolith limestone facies. The former is composed mainly of coral floatstone/framestone and the latter is characterized by rhodolith-large foraminifera floatstone/rudstone. By comparing the bio-sedimentary facies of the core samples with those of present sediments around Irabu Is., the coral limestone facies typically represents a reefal-shelf area of 0-60m water depth, and the rhodolith limestone facies an open shelf of 60-150m deep. The Ryukyu Group is widely distributed on Irabu Is., gently dipping west. It can be divided into three Members, and each separated from its neighbours by a solution unconformity. The Lower Member comprises coral framestone/floatstone and overlies the Shimajiri Group with an angular unconformity, which gradually dips westward. The Middle Member is characterized by five cyclic sequences of alternating coral limestone facies (Unit C-1, C-2, C-3, C-4, C-5) and rhodolith limestone facies (Unit R-1, R-2, R-3, R-4). However, the coral limestone facies dominates in the eastern hilly area but abruptly decreases in thickness and is increasingly replaced by the rhodolith limestone facies towards the western lowland. The Upper Member is a large foraminifera grainstone/packstone. Boreholes can be correlated on the basis of lithostratigraphy and calcareous nannofossil biostratigraphy. Five nannofossil-datum levels show the Ryukyu Group to be 1.36-0.39Ma. The Ryukyu Group accumulated in a reefal to shelf environment where the water depth was increasing toward the west. At low sea level stand, the coral limestone facies extended for a distance westward and at high sea level stand, the rhodolith limestone facies was distributed widely in the area. The unconformity between the Lower and Middle Members falls between 1, 10Ma and 0.89Ma. A number of unconformities, regressive sequences, and cooling episodes are recorded at this time in Japan. According to nannofossil-datum, the lowest rhodolith limestone facies of the Middle Member may be correlated with the oxygen isotope stages 25-23 of Williams et al. (1988), and the coral-rhodolith sedimentary cycles in the Middle Member are considered to be formed in response to glacial eustacy (oxygen isotope stages 26-16).
The depositional sequence in the Upper Jurassic Arab•Hith Formations, which is main reservoir zone of the Neewat Al Ghalan (GA) field, offshore Abu Dhabi, is composed of sabkha-nearshore deposits and divided into two systems tracts (transgressive systems tract (TST) and highstand systems tract (HST)) based on the characteristics of parasequence stacking patterns. Each parasequence in TST shows a shallowing upward cycle which commences from barrier shoal grainstone and terminates in lagoonal mudstone or sabkha anhydrite. Vertical stacking patterns of these parasequences within TST indicate thinning upward cycle and retrogradational/aggradational pattern. The parasequences in HST are characterized by a shallowing upward cycle from tidal channel grainstone or lagoonal mudstone to sabkha anhydrite. Stacking patterns of both bedsets parasequences within HST show overall thickening upward cycles and progradational pattern. The stacking patterns in the depositional sequence reflect various orders of sea level fluctuations. During TST deposition the barrier shoal systems accompanied by shoreface erosion developed in the field. In the succeeding HST deposition, the barrier shoal (strand plain) systems were migrating basinward, and the tidal flat-sabkha systems, in turn, were prograding to the field. These environmental changes due to the sea level fluctuations are considered to have affected the reservoir quality (cementation or dolomitization) of the field.