The Triassic Yanchang Formation is currently a hot spot for lacustrine shale-gas exploration in the Ordos Basin, China. A seismic-sedimentological study using a newly acquired 3D survey was conducted to advance the interpretation and prediction of the shale-gas reservoirs. The seismic volume was converted into a log-lithology volume at the thin-bed level. The inverted acoustic impedance volume was stratal sliced to extract seismic geomorphologic information. Deltaic facies and depositional history were interpreted using lithologic and planform morphology of seismic attributes on stratal slices. Detailed core analysis data including total organic content (TOC), mineral composition, and mechanical properties of sandstones and shales were correlated with seismic-derived lithology and facies. Microseismic data were evaluated for reservoir performance in production wells. Finally, shale-gas sweet spots, or optimal drilling targets, were predicted by mapping relatively high-brittleness lithofacies in 3D...

Seismic expression of delta to deep-lake transition and its control on lithology, total organic content, brittleness, and shale-gas sweet spots in Triassic Yanchang Formation, southern Ordos Basin, China

Southern Ordos Basin contains plentiful oil and gas shale resources in the Mesozoic and Upper Paleozoic. It is known that the surface in this region is covered by thick loess layers with severe variations in tomography, thickness and velocity. The adverse surface conditions result in complex statics problems and heavy scattering noise and also cause serious absorption of high-frequency energy. Conventional seismic surveys obtain poor quality seismic data, which could hardly be used to obtain information about gas shale reservoirs. In order to study strong heterogeneous gas shale reservoirs and improve the success rate of horizontal wells, a new 3D seismic survey was acquired in the typical loess plateau area by using the WesternGeco UniQ integrated point-receiver land seismic system. The new survey is designed as full-azimuth, high-density, point-source/point-receiver in order to fully sample the seismic wavefield and avoid the irreversible signal damage caused by conventional field array techniques. By t...

Full-Azimuth, High-Density, 3D Single-Point Seismic Survey for Shale Gas Exploration in a Loess Plateau Area, Southeast of Ordos Basin, China

seismic sequences to be distinguished in the tongue. Correla tion of reflectors with well data allows ages, ranging from mid dle Miocene to Holocene, to be assigned to the seismic se quences. Sedimentary processes responsible for deposition of each se quence are determined on the basis of external geometry, inter nal reflection configuration, and where possible, core data. Primary deposits consist of proximal and distal turbidites, and hemipelagites and pelagjtes. Sediments may be later modified by creep or slumping and sliding. This seismic stratigraphic study allows the depositional history of the Veracruz tongue to be elucidated and aids in the dating of folding and salt move ment at the edges of the tongue. Detailed interpretation of seismic waveforms indicates that gas accumulations are present in the study area. Future seismic stratigraphic studies will allow quantitative evaluation of hydrocarbon potential of the Veracruz tongue and the adja cent foldbelt.

on Sedimentology of

“Seismic exploration in complex near-surface and foothills regions – a case study from northwest of the Tarim basin”

In the study area, southeast of Ordos Basin in China, thick lacustrine shale/mudstone strata have been developed in the Triassic Yanchang Formation. Aiming to study these source/reservoir rocks, a 3D full-azimuth, high-density seismic survey was acquired. However, the surface in this region is covered by a thick loess layer, leading to seismic challenges such as complicated interferences and serious absorption of high frequencies. Despite a specially targeted seismic processing workflow, the prestack Kirchhoff time-migrated seismic data were still contaminated by severe noise, hindering seismic inversion and geologic interpretation. By taking account of the particular data quality and noise characteristics, we have developed a cascade workflow including three major methods to condition the poststack 3D seismic data. First, we removed the sticky coherent noise by a local pseudo f-x-y Cadzow filtering. Then, we diminished the random noise by a structure-oriented filtering. Finally, we extended the f...

Preconditioning point-source/point-receiver high-density 3D seismic data for lacustrine shale characterization in a loess mountain area

The Ordos Basin, located in the central part of China, contains abundant oil and gas shale resources. This study took place in an area southwest of Yan’an City, Shaanxi province, and its main exploration targets were the Mesozoic Yanchang Formation, with a maximum depth of around 1500 m, and the Upper Paleozoic Benxi Formation, with a maximum depth of around 3550 m. The tectonic structure at the target levels is almost flat, with dips of only around one degree.
The objective of the survey was to identify ‘sweet spots’ in a heterogeneous gas shale reservoir beneath thick deposits of loess, which is sediment formed by the accumulation of wind-blown silt. Variations in the topography, thickness, and seismic velocity of the loess were identified as key factors that had led to inadequate imaging results from previous 2D exploration surveys in the area (Yao and Li, 2004). The new 3D survey, acquired by CNPC Sichuan Geophysical Company (SCGC) during 2013 using the WesternGeco UniQ integrated point-receiver land seismic system, delivered imaging results that improved the characterization of the gas shale.
Several features of the project area presented challenges for seismic exploration, many of which are typical of a loess plateau.
The area is covered by unconsolidated loess with severe variations in thickness and velocity that can result in serious statics problems. The loess also causes dramatic signal absorption, leading to poor signal-to-noise ratio and potentially contributing to low resolution in the seismic data. As shown in Figure 1, surface conditions include rapid variations in topography that further complicate near-surface statics correction and can be the cause of various types of interference and noise. Survey logistics needed to account for heavily forested areas, hills and gulches. Also, more than 500 oil wells were pumping and some rigs were drilling during survey acquisition, representing additional sources of noise (Figure 2).

Full-azimuth, high-density, 3D point-source/point-receiver seismic survey for shale gas exploration in a loess plateau: a case study from the Ordos basin, China

Summary A seismic-sedimentologic study in a newly acquired 3D survey in the Erdos Basin, China, has advanced interpretation and prediction of thin sandstone reservoirs and shale units in the prolific Yanchang Formation. With much-improved seismic data, a high-resolution mapping of lithology, geomorphology, and depositional facies became possible. At a below-resolution level, the integrated study revealed various depositional systems, including deep-lake, deltaic, and fluvial systems on stratal slices, which reflect complex distribution of thin-bedded conventional reservoirs and potential shale-gas plays.

Applying sedimentary sedimentology to high-resolution depositional-facies mapping in Triassic Yanchang Formation, Erdos Basin, China

Facies control on lithology, TOC, and brittleness: Predicting lacustrine shale-gas sweet spots by using seismic data

Preconditioning single-point 3D seismic data for improved reservoir characterization in a Loess mountain area

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Full-azimuth, high-density, 3D point-source/point-receiver seismic survey for shale gas exploration in a loess plateau: a case study from the Ordos basin, China

Applying sedimentary sedimentology to high-resolution depositional-facies mapping in Triassic Yanchang Formation, Erdos Basin, China

Facies control on lithology, TOC, and brittleness: Predicting lacustrine shale-gas sweet spots by using seismic data

Preconditioning single-point 3D seismic data for improved reservoir characterization in a Loess mountain area