Hongliu Zeng

Stratal slicing, Part I: Realistic 3-D seismic model

Two-dimensional, fenced 2-D, and 3-D isosurface displays of some realistic 3-D seismic models built in the lower Miocene Powderhorn Field, Calhoun County, Texas, demonstrate that a seismic event does not necessarily follow an impedance boundary defined by a geological time surface. Instead, the position of a filtered impedance boundary relative to the geological time surface may vary with seismic frequency because of inadequate resolution of seismic data and to the en echelon or ramp arrangement of impedance anomalies of sandstone. Except for some relatively time-parallel seismic events, the correlation error of event picking is large enough to distort or even miss the majority of the target zone on stratal slices. In some cases, reflections from sandstone bodies in different depositional units interfere to form a single event and, in one instance, an event tying as many as six depositional units (interbedded sandy and shaly layers) over 50 m was observed. Frequency independence is a necessary condition for selecting time-parallel reference events. Instead of event picking, phantom mapping between such reference events is a better technique for picking stratal slices, making it possible to map detailed depositional facies within reservoir sequences routinely and reliably from 3-D seismic data.

Stratal slicing, Part II: Real 3-D seismic data

Three‐dimensional seismic data from the Gulf of Mexico Tertiary section show a close dependence of seismic events on data frequency. While some events remain frequency independent, many events exhibit different occurrences with changing frequency and, therefore, are not parallel to geologic time surfaces. In the data set we have studied, observed maximum time transgression of seismic events is at least 120 ms traveltime on lower frequency sections. Severe interference in lower frequency data may produce false seismic facies characteristics and obscure the true stratigraphic relationships. This phenomenon has important implications for seismic interpretation, particularly for sequence stratigraphic studies. This time transgression problem is mitigated to a large degree by the stratal slicing technique discussed in Part I of this paper. Stratal slicing on a workstation is done by first tracking frequency‐independent, geologic‐time‐equivalent reference seismic events, then building a stratal time model and a...

High-frequency sequence stratigraphy from seismic sedimentology: Applied to Miocene, Vermilion Block 50, Tiger Shoal area, offshore Louisiana

We introduce a seismic-sedimentological approach for mapping high-frequency (fourth-order) sequences and systems tracts using well and three-dimensional seismic data. Key techniques include (1) conditioning seismic data to log lithology by 90 phasing for better well-log integration and (2) imaging and interpreting the sequential, planoform geomorphology of the depositional systems. We recommend a new interpretation procedure that shifts the emphasis of high-frequency sequence-stratigraphic studies from interpreting vertical seismic sections to analyzing more horizontal, high-resolution, seismic-geomorphologic information.This case study shows that stratal slicing in lithology-conditioned seismic data provides sequential seismic imagery of generally contemporaneous depositional systems. This imagery, in turn, serves as a basis for recognizing and mapping high-frequency systems tracts, sequence boundaries, and sequences in a geologic-time domain. In Miocene strata of offshore Louisiana, fourth-order sequences or sequence sets from well data can be seismically mapped at a resolution equivalent to 30 ft (10 m) in thickness from a 30-Hz dominant-frequency seismic data set. The resolution is sufficient for an accurate reconstruction of the high-frequency sequence-stratigraphic framework in the region of seismic coverage outside well control.

Interpretive advantages of 90°-phase wavelets: Part 1 — Modeling

We discuss, in a two-part article, the benefits of 90°-phase wavelets in stratigraphic and lithologic interpretation of seismically thin beds. In Part 1, seismic models of Ricker wavelets with selected phases are constructed to assess interpretability of composite waveforms in increasingly complex geologic settings. Although superior for single surface and thick-layer interpretation, zero-phase seismic data are not optimal for interpreting beds thinner than a wavelength because their antisymmetric thin-bed responses tie to the reflectivity series rather than to impedance logs. Nonsymmetrical wavelets (e.g., minimum-phase wavelets) are generally not recommended for interpretation because their asymmetric composite waveforms have large side lobes. Integrated zero-phase traces are also less desirable because they lose high-frequency components in the integration process. However, the application of 90°-phase data consistently improves seismic interpretability. The unique symmetry of 90°-phase thin-bed response eliminates the dual polarity of thin-bed responses, resulting in better imagery of thin-bed geometry, impedance profiles, lithology, and stratigraphy. Less amplitude distortion and less stratigraphy-independent, thin-bed interference lead to more accurate acoustic impedance estimation from amplitude data and a better tie of seismic traces to lithology-indicative wireline logs. Field data applications are presented in part 2 of this article.

Three-dimensional seismic geomorphology and analysis of the Ordovician paleokarst drainage system in the central Tabei Uplift, northern Tarim Basin, western China

High-quality three-dimensional seismic data acquired in the central Tabei Uplift, Tarim Basin, western China, provide a rare opportunity to characterize in exceptional detail the three-dimensional geomorphology of a deeply buried (5500–6500 m [18,045–21,325 ft]) Ordovician unconformity and the related paleokarst drainage system. An integrated approach was applied that emphasized integration of seismic data with available conventional core, wireline logs, and age-equivalent outcrops. The exceptional quality of the seismic data allowed a seismic detection limit of karstified features of less than 75 75 m (246 246 ft) horizontally and 6 m (20 ft) vertically. Interpreted geomorphologic and depositional elements include fluvial channels and canyons, fluvial valleys, sinkholes, and tower karsts and hills. The modern tower karst-drainage system in Guilin, China, is very similar to the mapped Ordovician karst-drainage system and is used as a modern analog. The interaction between the surface karst-drainage system and the shallow-subsurface cave-passage system is evidenced by the observation that surface canyons appear to initiate in areas associated with intense sinkhole development. Also, surface river valleys tend to correspond to dip-oriented surface depressions partly related to near-surface cave collapse. During burial into the deeper subsurface, the combination of intrastratal collapse (karstified strata) and suprastratal collapse (postkarst-deposited strata) created large damage zones hundreds of meters thick and kilometers wide. Coalesced-collapsed paleocave systems can be interpreted from the unique circular pattern of faults (observed in map view) that are associated with seismic bright spots.

Interpretive advantages of 90°-phase wavelets: Part 2 — Seismic applications

We examine field seismic data to test the benefits of 90°-phase wavelets in thin-bed interpretation that are predicted by seismic modeling in part 1 of this paper. In an interbedded sandstone-shale Miocene succession in the Gulf of Mexico basin, a 90°-phase shift of nearly zero-phase seismic data significantly improves lithologic and stratigraphic interpretation. A match between seismic and acoustic impedance (AI) profiles results in a better tie between seismic amplitude traces and lithology-indicative logs. Better geometric imaging of AI units that does not use dual-polarity seismic events results in easier and more accurate reservoir delineation. Less amplitude distortion and the stratigraphy-independent nature of thin-bed interference significantly improves stratigraphic resolution and seismic stratigraphic profiling. For a Ricker-like wavelet having small side lobes, stratigraphic resolution of 90°-phase data is considerably higher than that of zero-phase data. In this specific case, stratigraphic re...

Stratal slicing of Miocene-Pliocene sediments in Vermilion Block 50-Tiger Shoal Area, offshore Louisiana

Many people have viewed modern land surfaces from commercial airplanes and marveled at the form and geometry of geomorphic features such as river channels, deltas, barrier islands, and dune fields. These views represent complete images of the modern time surfaces. We can classify the depositional nature of features on these images by interpreting their planiform geometry and geographic context. In fact, modern 3-D seismic technology has made it possible for us to image similar, but much older, geomorphic or depositional features preserved in the rock record. Unfortunately, although many reservoir-scale (well-to-well scale) features can be detected in vertical seismic lines, few such features can be resolved and interpreted in the vertical perspective because of the datas limited bandwidth. Only in the horizontal perspective are such depositional features large enough to be resolvable when displayed in map view on geologic time surfaces.

Seismic frequency control on carbonate seismic stratigraphy: A case study of the Kingdom Abo sequence, west Texas

Conceptual models and real three-dimensional (3-D) seismic data show that in progradational carbonate platform margin and slope deposits of the Kingdom Abo reservoir of the Permian basin, west Texas, primary seismic reflection events do not necessarily follow clinoformal geologic-time surfaces. The seismic frequency content of the data controls the dip and architecture of seismic reflection events. High-frequency seismic data tend to follow thinner, time-bounded clinoform depositional elements (time-stratigraphic units), whereas low-frequency seismic data tend to image thicker, low-angle lithofacies units (time-transgressive units). In seismic data of moderate frequency, both clinoform units and flat lithofacies units are imaged, creating complex interference patterns that are difficult to interpret.Experiments with models and real data demonstrate that seismic data can be selectively filtered in the signal bandwidth to help distinguish time-stratigraphic units from lithostratigraphic units. Selective filtering alters the dominant frequency of the data to match a desired scale of geologic objects. If there are enough high-frequency components in the seismic data, true clinoform stratigraphy can be imaged even if the data are dominated by lower frequency components.Seismic modeling of outcrop of the Abo sequence in Apache Canyon, Sierra Diablo, west Texas, indicates that a dominant frequency of 100 Hz is needed to recover true clinoform stratigraphy using seismic data. The interpretation of available 3-D seismic data can only partially distinguish time-stratigraphy from lithostratigraphy because of the lack of frequency components greater than 70 Hz in the data. Application of this outcrop model in seismic modeling avoids interpretational pitfalls that can occur if the dominant frequency of the seismic data is not matched to the unit thicknesses that need to be resolved.

Guidelines for seismic sedimentologic study in non-marine postrift basins

Abstract This study summarizes the research experiences of non-marine seismic sedimentology in recent years in China and uses Qijia area, Songliao Basin, as a template to establish general guidelines for seismic sedimentology. Basic data sets include stacked 3D seismic volumes, 2D regional seismic lines, data for regional geologic settings, and well data. The workflow emphasizes the integration of seismic and geologic interpretations and balanced use of seismic sedimentology, sequence stratigraphy and seismic stratigraphy. Basic steps include well-to-seismic tie for the establishment of sequence framework, wavelet-phase adjustment, picking of geologic-time parallel seismic events, seismic resolution analysis, petrophysical analysis, selection of seismic attributes, stratal slicing, seismic depositional facies analysis, and applications to exploration and development. Expected maps range from key interpreted well-seismic sections, flattened relative geologic-time sections, stratal slices, and depositional facies maps, etc. The workflow is applied in the study of the Cretaceous Qingshankou Formation in the Qijia area, Songliao Basin, which can be used as a reference for seismic sedimentologic study in non-marine basins, especially in postrift depression-type basins in China.

Mapping sediment-dispersal patterns and associated systems tracts in fourth- and fifth-order sequences using seismic sedimentology: Example from Corpus Christi Bay, Texas

A seismic-sedimentologic study was performed to map fourth- and fifth-order systems tracts in Oligocene (Frio) strata in Corpus Christi Bay, south Texas. Guided by third-order sequence-stratigraphic correlations from seismic and wire-line-log data, we prepared stratal slices from a three-dimensional seismic volume to reveal high-resolution (10-m [33-ft] levels) sediment dispersal patterns and associated systems tracts in a relative geologic-time domain. On average, 1200 m (3940 ft) of sediments were deposited in the third-order lowstand expansion cycle, and at least 16 higher order sequences (fourth- and fifth-order sequences) were recognized. Three types of depositional systems were identified in the Frio stratigraphic section: (1) offshelf lowstand slope fans that are best characterized by submarine channel and levee systems inside and outside incised submarine channels and by fan-shaped sand body geometry; (2) lowstand prograding deltaic systems that are composed of strike-oriented and lobate deltaic sand bodies; and (3) highstand systems that are represented by onshelf barrier, lagoon, and deltaic systems. Higher order sequence development was controlled by the interaction of relative sea level change, sediment supply, and gravity tectonics. The top of sediment ridges was eroded or decapitated during many of the higher order sequences. Sand dispersal patterns are primarily controlled by accommodation resulting from rollover topography associated with growth faulting. Between the boundary fault and the hinge line atop rollover structures, strike-oriented sandstone bodies dominate; within submarine channels and incised valleys and beyond the hinge line to the distal basin, dip-oriented sandstone bodies prevail. Sandstone thickness and dispersal patterns can be predicted by integrating wire-line-log measurements and seismic amplitude patterns.