This paper comprehensively reviews recently discussed sedimentological and petroleum geological topics on submarine-fan turbidites as the major hydrocarbon reservoir rocks of the world. After the proposals of primary basic submarine-fan models in 1970', many academic and industrial survey results recognized that submarine-fan depositional configuration has much variation affected by various control factors. Although practical type classification has been proposed, “sandy radial fan type” and “muddy channel levee type” can be recognized as the two representative configuration types of submarine fans. “Hyperpycnal flow” is a newly recognized flow mechanism to transport a certain amount of terrestrial organic materials into the deep sea territory, resulting in the formation of high TOC source and reservoir rocks of turbidites. Submarine-fan turbidites occur not only in a clastic deep-sea environment but also on a continental shelf as “shelf turbidites”, in a carbonate setting as “calci-turbidites” and in a shale basin as calci- or silty turbidites, all of which possess the potential as exploration targets. Reservoir property of turbidite sandstones can be controlled by three basic factors: background, primary facies and secondary diagenesis factors. Coarser facies deposited in a channel or depositional lobe tends to demonstrate high reservoir quality primarily but easily be cemented if diagenetic effect is intense. As the major trap type of turbidite sandstone reservoirs, the combination of structural and stratigraphic traps can easily be found in the world-wide oil fields. “Seismic geomorphology” using 3D seismic survey data has widely been used for detailed distribution and property analysis of submarine-fan tubidites, as a recently developed effective analytical method. The obtained data are provided to a geostatistical reservoir modeling as basic geologic framework constraints. To implement sedimentologically meaningful reservoir modeling, object-based modeling, multipoint-geostatistics using a training image, surface-based modeling and depositional process-based modeling are adoped for submarine-fan turbidite reservoirs.

It has been known that submarine fans are diverse in constitutive architectural elements. Leveed channels are common architectural elements of submarine fans but some of submarine fans lack such self-constructive channels. Also, frontal splays that form radial shape of submarine fans do not always develop at the end of submarine channels. Several researches suggested that this diversity of submarine fans is originated from attributes of sediment supply such as ratio between sand and mud. Here this study aims to consider deterministic parameters that govern types of submarine fans, using a theoretical model that assumes submarine channel are in equilibrium condition. The equilibrium profile is here defined as profiles where turbidity currents are bypassing or depositing uniformly. 1D momentum conservation is considered, and grain-size distribution is approximated to two size classes : channel-forming sands and levee-forming muddy sediments. Widths of channel and levees are calculated from empirical hydraulic geometry. The model predicts length of leveed channels in the equilibrium condition and hydraulic conditions at downstream end of the channels. As a result, depending on boundary condition at upstream end of channels, two possible cases were recognized: (i) submarine channels in which levee-forming muddy sediments are run out at the end of channel, (ii) submarine channels in which sandy sediments are run out. It can be interpreted that these types of results correspond to submarine channels with/without frontal splays. Essentially three parameters are significant to determine types of submarine channels : sand/mud ratio of supplied sediments, discharge of turbidity currents and rate of aggradation of channels. Although application of this research is reconnaissance, this model may be useful to estimate flow conditions from topographic features of submarine fans and distribution of lithofacies from limited information.

This paper presents the following three topics among new findings on reservoir architecture and hydrocarbon potential of submarine channel - levee - fan systems.

Firstly, this paper summarizes characteristics and origin of new intra-channel reservoir architecture “outer-bank bars (OBBs)”. OBBs develop on the outer banks of meander bends of sinuous submarine channels and dip towards the inner banks, a direction of dip opposed to that of point bars or lateral-accretion packages. This paper then discusses possible reservoir quality of OBBs and possibilities of occurrence of OBBs in fluvial channels.

Secondly, this paper reviews a recent study in the geometry of submarine levees. The variation in levee thickness perpendicular to the channel shows power-law decay on steeper slopes (>0.6°) and either exponential or logarithmic decay on gentler slopes. Grain size may also control the shape of levees. A threshold in slope gradient is here interpreted as the boundary between subcritical and supercritical flows within turbidity currents flowing over levees. Based on the geometry of submarine levees, new criteria to distinguish sand-prone levees from mud-prone levees are proposed.

Finally, a new facies model of hyperpycnites is presented based on the “rhythmite” beds found on the lobe and the levees of the Toyama Deep Sea Channel in the central Sea of Japan. Hydrocarbon potential of hyperpycnites are also examined by organic carbon analysis of them. On the basis of proposed criteria to identify hyperpycnites, two potential fields of hydrocarbon; the Kutei Basin offshore East Kalimantan and the Niigata-Shin'etsu Basin to Toyama Trough, where hyperpycnites may constitute hydrocarbon source rocks, are suggested. In the latter case, a new hydrocarbon source model, which can explain origin of the “Kubiki type oil”, is presented in this paper.

Sedimentary forward modeling is an effective and economical approach for exploration of sandstone reservoirs, especially in deepwater settings. Computational fluid dynamics brought innovation to modeling and simulating the behaviors of sediment gravity flows (turbidity currents in most cases) along with the prediction of turbidite distribution pattern. Furthermore, establishment of structural and isostatic balancing technique contributed to quantitative reconstruction of paleo-basin structure and paleo-bathymetry. Recent dramatic improvements of computer operating speed and software performance have allowed us to apply such numerical methods to a three-dimensional (3-D), several ten to hundred kilometer wide dataset. The integration of these sedimentary and structural approaches has finally produced an advanced forward sedimentary model that numerically predicts a turbidite distribution on the reconstructed 3-D paleo-bathymetry. Such an integrated model is powerful and useful for better understanding of lobate turbidite distribution and of turbidite deposition in topographically complicated basins, or confined basins, and also for stochastic determination on input parameters of turbidity current (s) based on a limited number of available well-log data. Combination with seismic geomorphology and seismic attribute analysis can improve the quality and accuracy of geological evaluation in deepwater turbidite play.

An extremely low velocity layer was detected in a deep water area, whose velocity was estimated to be 1500 m/s by velocity analysis of the seismic data. Its thickness is approximately 500 m in average and exceeds 700 m in maximum. Although obtained data are limited to reveal its origin, the layer was interpreted to be a mass transport deposit based on the observations: a distribution feature spreading toward the deep water, a chaotic-to-transparent reflection pattern, basal erosions of the underlying sediments and a pressure ridge developed at the tip region. However, mass transport deposit is normally characterized by high seismic velocity according to previous studies, whereas such a mass transport deposit with low velocity is seldom reported. From detailed seismic interpretation, active normal faults cutting the seafloor were recognized in overlying sediments. The faults appear also to cut the uppermost part of the mass transport deposit. Moreover, pore pressure gradient within the deposit was estimated to be from 12.0 to 14.4 MPa/km, being comparable with the fracture gradient from previous study. As a result, the presently-detected low velocity layer can be concluded most likely as a slow-moving earthflow type of mass transport deposit in deep water.

In the early 1970's, oil and gas exploration was started in the west of Shetlands, United Kingdom, which is a part of the Atlantic Margin.

One of the main exploration targets in the west of Shetlands is the Paleocene deepwater sandstones, some discoveries of which proceeded to development or production stages. The Laggan field discovered in 1986 is under development together with a neighboring discovery, Tormore field. The Foinaven field has been producing oil since 1997. These Paleocene fields are characterised by seismic attributes including amplitude and AVO anomalies, and understanding of these attributes enables reduced geological risks and estimation of reserves, not only in exploration but also in development and production stages.

On the other hand, there are many failures of exploration wells in areas with seismic anomalies. Loizou (2005) examined all the exploration wells in the west of Shetlands drilled until 2005, and concluded that only 9 out of 39 wells, which have targeted Paleocene seismic anomalies, resulted in geological success. To improve the success-to-failure ratio in exploration associated with seismic anomalies, it is important to construct a plausible geological model including trapping mechanism, seal, reservoir, source rock and hydrocarbon migration that is consistent with hydrocarbon accumulations implied by seismic anomalies.

The Tornado discovery was a prospect, which targeted the Paleocene deepmarine sandstone associated with a seismic amplitude anomaly. The first exploration well confirmed a gas column with oil leg. Prior to the drilling, seismic data analysis was carried out together with a CSEM data acquisition. Those geophysical information was integrated with a geological model built on the information from nearby wells and other geological studies to reduce geological risks (Pickering et al., 2011). This paper presents successful examples of integrated methods of exploration activities which do not rely only on seismic anomalies.

The Ichthys gas-condensate field is located in the Browse Basin, North West Shelf of Australia and it is currently under development.

The Brewster Member, one of the main reservoirs of Ichthys field consists of submarine channel and lobe complex sandstones and minor mudstones, approximately 200 m thick, and has a high net sand-to-gross thickness ratio (over 90%).

Although massive sand facies dominates and porosity is relatively stable at around 10%, the permeability distribution is complicated. Core measurement indicates that permeability has a large variation against porosity. In addition, an abrupt reduction of permeability with depth is observed in all wells in the field. The depth of the permeability reduction in each well can be mapped as a gently dipping almost planar surface. The surface, which is a potential diagenetic boundary separating good and poor permeability reservoirs, is called the Diagenetic Limit (DL).

To investigate the cause of DL, detailed petrographic analysis and Illite morphology observation were conducted.

The results of these analyses indicate that:

· “Quartz cement volume per Intergranular volume” and Illite volume have negative correlations with permeability.

· Illite volume increases with reservoir depth.

· Dominant Illite morphology changes from “flaky” to “fibrous” with reservoir depth. It becomes fibrous dominant in all observed samples below the DL.

Based on these results, two ideas for the cause of the DL have been developed:

1) Differential diagenesis occurred due to the existence of an initial hydrocarbon accumulation, which may have inhibited or reduced the level of diagenesis in the interval above the paleo-FWL.

2) Differential diagenesis was inherited from differences in texture and mineralogy of primary deposits.

Both hypotheses still have issues to be verified. Since the prediction of permeability distribution is essential to maximize the hydrocarbon recovery from the field, further investigation will be conducted to clarify the cause of the DL.

To obtain basic information for methane hydrate (MH) reservoir characterization at the first offshore production test site (AT1) located on the northwestern slope of the Daini-Atsumi Knoll in the eastern Nankai Trough, extensive geophysical logging and pressure coring using a hybrid pressure coring system were conducted in 2012 at a monitoring well (AT1-MC) and a coring well (AT1-C).

The MH-concentrated zone (MHCZ), which was confirmed by geophysical logging at AT1-MC, has a 60-m-thick turbidite assemblage with sublayers ranging from a few tens to hundreds of centimeters thickness. The turbidite assemblage is composed of lobe/sheet-type sequences in the upper part and relatively thick channel-sand sequences in the lower part. Well-to-well correlations of sandy layers between two monitoring wells (AT1-MC and MT1) within 40 m of one another exhibited fairly good lateral continuity of sand layers in the upper part of the reservoir. This suggests an ideal reservoir for the production test. On the other hand, in the lower part of the MHCZ, each sand layer varies in thickness and there are difficult to correlate individually. This means sand continuity in the lower MHCZ is not necessarily as good as in the upper MHCZ. In the MHCZ, MH pore saturation (Sh) values of 50% -80% were observed from geophysical logging in sandy layers, which is in fairly good agreement with core-derived Sh values.

These reservoir information were utilized to determine production interval and history matching as well. We are expecting to use these data and knowledge not only for next offshore MH production test preparation but also other conventional oil and gas projects.