High-resolution 3D Seismic Surveys Research Articles

紀伊半島南東沖南海トラフでの高分解能3次元反射法地震探査と重合前深度マイグレーション処理

IODP南海トラフ地震発生帯掘削計画として紀伊半島沖東南海地震（マグニチュード8.1）の震源域掘削が2007年秋頃から始まる。それに先立ち，我々は2006年3月，紀伊半島南東沖南海トラフ付近における地殻構造の高精度イメージングのため，深海調査研究船「かいれい」のマルチチャンネル反射法地震探査システムを用いた高分解能3次元反射法地震探査を行った（KR06-02航海）。「かいれい」3次元探査域は3ヶ所の掘削サイトをカバーしており，本調査には長さ約5kmのストリーマーケーブル（204チャンネル）と，約100 m 離れた2式の震源アレイを用いた。高分解能調査のため用いた各々の震源アレイはGガン2基とGIガン1基の組合せである。特に，ストリーマーケーブル1本のみを曳航する本調査では，左右震源アレイを交互に発震するFlip-flop方式を導入することで，1 sail line につき2 CMP line のデータ取得が可能となり，データ取得作業の効率が倍増した。最終的な3次元データ取得範囲は3.5×52 km となった。データ記録長は10秒,サンプリング間隔は1 msec である。また，震源アレイとストリーマーケーブルの曳航深度は，それぞれ5mと8mに制御した。発震点および受振点の測位のため, SPECTRAとREFLEXを使用した。調査期間中に船上QCなどの結果，良好なデータ取得が確認できた。調査終了後，陸上での3次元ビンニングなどの前処理を終えたCMPデータを用い，3次元重合前深度マイグレーション処理を行った。最終的に，3次元区間速度モデルと高分解能の地殻構造イメージが得られた。速度不確定性を推定するために行った3D PSDM速度テストの結果より，最終速度モデルは，約6kmの深度において最大±5％の速度不確定性を持つことがわかった。得られた3次元地殻構造の解釈の結果，南海トラフ底で沈み込んでいる，3つの音響ユニットから成る四国海盆堆積層の層厚変化が明らかとなった。特に，最上位のユニットCは，トラフ底から陸側への有意義な層厚増加や背斜構造によって特徴付けられ，また，ユニットCの中央には強振幅の反射面Rの存在が認められる。この反射面Rは斜めスリップ断層面として解釈され，このスラスト断層運動によって，ユニットCが重なり，陸側へ厚くなっていることが考えられる。

Pliocene paleoenvironment evolution as interpreted from 3D-seismic data in the southern North Sea, Dutch offshore sector

A high-resolution 3D-seismic survey from the Dutch offshore sector has been interpreted and subsequently correlated with existing regional seismo-stratigraphic concepts derived from conventional 2D-seismic data sets. The interpreted 13 seismic units have been related to a newly established chrono-stratigraphic framework [Kuhlmann et al., 2006a, b. Chronostratigraphy of Late Neogene sediments in the southern North Sea Basin and paleoenvironmental interpretations. Palaeogeography, Palaeoclimatology, Palaeoecology, 239, 426–455; Integrated chronostratigraphy of the Pliocene–Pleistocene interval and its relation to the regional stratigraphical stages in the southern North Sea region. Netherlands Journal of Geosciences-Geologie en Mijnbouw, 85(1), 19–35] resulting in up-dated age control for the seismic units. The generation of amplitude maps, time slices and isopach maps from the 3D-seismic data enabled detailed spatial and temporal reconstruction regarding the paleoenvironmental and climatological development as depicted by Kuhlmann et al. [2006a. Chronostratigraphy of Late Neogene sediments in the southern North Sea Basin and paleoenvironmental interpretations. Palaeogeography, Palaeoclimatology, Palaeoecology, 239, 426–455]. The lowermost seismic units S1–S4 comprise condensed Middle Miocene to Piacencian sediments, deposited under warm open marine conditions. These sediments show a uniform seismic facies of low-amplitude reflectors. The boundary of seismic unit S4–S5 (around 2.6 Ma) delineates a shift towards generally colder climate conditions that are connected to the onset of Northern Hemisphere Glaciation. Seismic unit S5 includes alternations of warmer and colder periods. During warmer periods, bottom currents generated elongated structures (2.5–4 km long, 300–500 m wide) on the horizon display. These layers show as well shallow gas accumulations with a more regional extent and are related to coarser-grained sediments sealed by clayey sediments of the cold phases. A homogenous seismic facies is characteristic for the colder periods. Within seismic units S6 and S7, internal fore set structures display the regressional trend related to the cooling of this interval. Seismic units S8–S11 display a pattern of unoriented lineaments which was probably caused by icebergs drifting into the North Sea and souring the sea floor. This finding is in accordance with the shallow marine, arctic conditions of this interval. Within these units a bright spot of 3 by 7 km in size, is interpreted to result from shallow gas accumulation related to a grounded iceberg. This feature represents another, more local area of gas accumulation that is closely related to iceberg activity within the study area. The uppermost seismic units S12 and S13 belong to a fluvial, paralic paleo-environment, that is characterised by aggradational seismic reflectors. Pleistocene glacial valleys were recognised in the upper 200 m below sea floor.

Fusion of 3D seismic exploration and seafloor geochemical survey for methane hydrate exploration

The MH21 Research Consortium has conducted a high-resolution 3D seismic survey and a seafloor geochemical survey, to explore methane hydrate reservoirs in the eastern Nankai Trough, offshore Japan. Excellent geological information about shallow formations was obtained from the high-resolution 3D seismic survey, which was designed to image the shallow formations where methane hydrates exist. The information is useful in constructing a geological and geochemical model, and especially to understand the complex geology of seafloor, including geochemical manifestations and the structure of migration conduits for methane gas or methane-bearing fluid.By comparing methane seep sites observed by submersibles with seismic sections, some significant relationships between methane hydrate reservoirs, free gas accumulations below the seafloor, and seafloor manifestations are recognised. Bathymetric charts and seafloor reflection amplitude maps, constructed from seismic reflections from the seafloor, are also useful in understanding the relationships over a vast area. A new geochemical seafloor survey targeted by these maps is required.The relationships between methane hydrate reservoirs and seafloor manifestations are becoming clearer from interpretation of high-resolution 3D seismic data. The MH21 Research Consortium will continue to conduct seafloor geochemical surveys based on the geological and geochemical model constructed from high-resolution 3D seismic data analysis.In this paper, we introduce a basis for exploration of methane hydrate reservoirs in Japan by fusion of 3D seismic exploration and seafloor geochemical surveys.

The architecture and evolution of the Middle Bengal Fan in vicinity of the active channel–levee system imaged by high-resolution seismic data

High-resolution seismic data from the Middle Bengal Fan were analysed to study the architecture of the fan in the vicinity of the active channel–levee system and to evaluate channel behaviours. The upper 600 m column of the fan in the study area reveal a pattern of different sized channel–levee systems and high amplitude reflection packets (HARPS) of varying thicknesses, no mass-flow deposits or hemipelagic drapes are found. Within the channel–levee systems, units of chaotic high amplitude reflectors (CHARS), which represent aggraded and migrated channel axes, as well as abandoned channel-fill deposits appear. All channels are characterised by erosional incision into underlying deposits. Within the active channel–levee system, numerous cut-off loops are found showing a broad spectrum of complex behaviours with different ratios of vertical aggradation and lateral migration. As a difference to the Amazon Fan, the floors of all cut off loops and the active channel are not elevated above the surrounding seafloor, which may lead to such a long lifetime resulting in the large number of cut-off loops. In contrast, a detailed studied buried system shows no cut-off loops and is characterised by a more simple behaviour: a first phase of lateral migration is followed by a second phase of simultaneous vertical aggradation and lateral migration, which finally elevated the channel floor above the level of the surrounding seafloor. These architectural differences between the systems indicate that different loaded turbidity currents have built up both systems. The aggrading and migrating channel floors likely have reservoir potential, but a high-resolution 3D seismic survey and drilling are necessary for final proof.

High-resolution 3D seismic imaging in practice

Pearl Oil (Thailand) acquired a high-density, high-resolution 3D seismic survey in 2004 to establish an accurate geologic framework for development drilling of the Jasmine oil field in block B5/27, offshore Thailand (Figure 1).

SEISMIC TO SIMULATION: INTEGRATED RESERVOIR MODEL FOR THE PATRICIA BALEEN GAS FIELD

The Patricia and Baleen offshore gas fields are located in the northeastern part of the Gippsland Basin in southeast Australia. Although discovered by two exploration wells almost a quarter of a century ago, the two gas fields only recently have again become the focus of appraisal and subsequent development activity through OMV’s acquisition of Cultus in 1999.After the drilling of a successful appraisal well in late 1999, a high resolution 3D seismic survey was acquired in early 2000. No further data acquisition will be undertaken. Special emphasis was therefore put on maximising the value of the 3D dataset by integrating the PreSTM (Pre. Stack Time Migration) seismic and several Elastic Impedance attributes with all other available subsurface data prior to building a sophisticated stochastic reservoir model for simulation.This paper describes how the integration of leading edge seismic technology with unconventional geological modelling was used to overcome a number of major challenges in order to build a coherent static reservoir model and constrain resource uncertainty given the limited amount of wireline and core data:A large proportion of the gas fields is strongly affected by seismic tuning which would introduce significant uncertainties on GRV and GWC estimations from seismic, if not accounted for properly. Likewise all seismic and to a somewhat lesser extent basic inversion based attributes used for reservoir property determination are strongly affected by this geophysical artefact: These challenges (and seismic pitfalls) were met by inverting the conventional 3D seismic for Pand S- wave impedances and generating a set of Elastic Impedance Cubes, difference cubes and LRM Cubes (standing for the elastic constants Lambda (λ), Rho (ρ) and Mhu (μ)), defining petroacoustic properties of the reservoir rocks. These cubes were tested for mathematical dependency and used for the conditioning of the facies and porosity models.The glauconitic Gurnard reservoir contains a high fraction of conductive minerals and is almost completely bioturbated. Conventional saturation estimations based on wireline-logs and conventional sequence stratigraphic facies description did not deliver a reliable picture: Instead a facies model based on ichnofabric analysis was built and constrained with data available at the three well locations. Saturation height functions were applied separately for each facies type. The Rho-Lambda (ρλ) cube was used to condition facies distribution away from the wells.More specifically, the results presented in the paper are:Elastic Impedance inversion provided vertical seismic resolution in the order of 4 m to 10 m, thereby allowing a more accurate seismic estimation of GRV and the GWC. Lamesf Constants were extracted from seismic in order to classify lithology.A realistic facies model was built utilizing the Rho- Lambda (ρλ) cube combined with ichnofabric analysis tied to permeability and water saturation distributions.Elastic Impedance Difference cubes were successfully calculated to eliminate tuning even further and condition the stochastic porosity model.Connected volume maps were used to optimise the production well pathsThe GIIP upside volume has been upgraded compared to that based on an earlier simplistic geological reservoir model used for simulation. A more realistic P10/P90 reserves range is now supported by a number of deterministic and stochastic reservoir models.

Abstract: A High Resolution 3D Seismic Survey Using 55 x 55 Feet Bin Size in Osage County, OK to Delineate Details of of Structure, Stratigraphy and Subtle Lithologic Variations

Abstract: A High Resolution 3D Seismic Survey Using 55 x 55 Feet Bin Size in Osage County, OK to Delineate Details of of Structure, Stratigraphy and Subtle Lithologic Variations

High-resolution 3D seismic reflection and coring techniques applied to late Quaternary deposits on the New Jersey shelf

Studies of variable, small-scale features typical of Quaternary glacimarine environments require an interdisciplinary approach: precisely navigated, high-resolution 3D seismic surveys nested within a regional 2D seismic framework and integrated with a precisely navigated coring program designed to sample specific shallow acoustic facies. We illustrate this approach with examples from continuing studies of sediments on the New Jersey continental shelf. Surficial stratigraphy associated with the latest Pleistocene-Holocene deglaciation has been imaged twice in true 3D using the Huntec® DTS system, a deep-towed, high-frequency (500–3500 Hz) boomer source. In both cases, nominal profile spacing was 10 m (2.5 m shot points). Penetration of ~40 ms (~30 m) was achieved. Seismic facies in the upper 5 m have been interpreted using differential GPS-navigated vibra-cores. Accuracy of position of both seismic profiles and core locations approaches 5 m, much smaller than the wavelengths of the geologic features being imaged/sampled. Survey results show multiple, sediment-filled channels buried beneath the modern outer shelf meandering south and southeast, in the general direction of the shelf edge. Channels are typically a few meters deep and 100–200 m across. Lateral continuity between the channeled surfaces within the outer-shelf wedge and the mid-shelf cannot be demonstrated because of later erosion, but both sets of channels incise reflector ‘R’, believed to be a pre-Late Wisconsinan exposure surface. AMS 14C dates on benthic foraminifers from cores show that channels on the midshelf are cut into sediment >45,000 yrs old, and filled with muddy sediment as young as ~12,500 yrs, whereas channels on the outer shelf are filled with coarser, and perhaps older, sediment. Both sets of filled channels are buried beneath a surficial sand layer. The integrated approach described here can remove some uncertainties and considerably ‘sharpen’ the Quaternary stratigraphic picture. Without the perspective obtained from 3D seismic surveys and equally precise sampling, interpretations based on broader regional seismic surveys and spot coring are inevitably constrained by the potential for mismatching shallow acoustic and sedimentary facies.