Recognition Of Sequence Boundaries Research Articles

Sequence Stratigraphy of Well DX, Gongola Sub-Basin, Upper Benue Trough, Nigeria

: Biostratigraphic study of Well DX has yielded Cretaceous miospores and dinoflagellates cysts which heightened the recognition of sequence boundaries (SB), Maximum Flooding Surfaces (MFS) and associated Systems Tracts. Five maximum flooding surfaces between 95.6 Ma and 89.0 Ma, four sequence boundaries between 96.4 Ma and 93.0 Ma and threedepositional sequences were identified with varying average thicknesses of sediments interpreted from the gamma ray log and biostratigraphic data. The threedepositional sequences interpreted are -depositional sequence I (96.4 Ma - 95.4 Ma) (8240 ft. - 8120 ft.), depositional sequence II (95.4 Ma - 94.0 Ma) (8120 ft. - 7850 ft.) and depositionalsequence III (94.0 Ma - 93.0 Ma) (7850 ft. - 7550 ft.). All the depositional sequences fall within the third order cycle. The age of the well was attempted based on the presence of some selected marker fossils - Ephedripites spp., Classopollis spp., Spiniferites spp., Cyclonephelium distinctum, Cyclonephelium vannophorum, Subtilisphaera spp., Eucomiidites spp., Triorites africaensis, Odontochitina costata and Droseridites senonicus recovered from the studied intervals and was dated Albian - Santonian. The Sequence stratigraphic interpretations are useful in further deepening the knowledge of thesubsurface geology of the studiedwell in Gongola Sub Basin, Upper Benue Trough of Nigeria.Keywords: Sequence Boundary, Maximum Flooding Surface, System tracts, Depositional sequence

Use of Sedimentary Structures in the Recognition of Sequence Boundaries in Upper Jurassic–Upper Cretaceous Peritidal Carbonates of the Western Taurides, Turkey

Subtidal, intertidal, and supratidal carbonate facies are recognized in the Upper Jurassic (Kimmeridgian)-Upper Cretaceous (Cenomanian) peritidal carbonates of the Fele area (Western Taurides, Turkey). Vertical stacking patterns of these facies are of a cyclical character; shallowing upward is the trend of the cyclicity in these carbonate facies. In-situ karstic breccias, collapse breccias, caliche (laminar calcrete), “Microcodium” accretion, and root casts are structures commonly indicative of third-order sequence boundaries. However, mud cracks, solution pores or vugs, sheet cracks, loferites, and birds-eye structures are commonly delineated by parasequence boundaries. In-situ or collapse breccias can be genetically derived from sheet cracks, mud cracks, solution pores or vugs, and birds-eye structures with increasing exposure time. The use of such sedimentary structures in the recognition of sequence boundaries is highly practical in the ancient carbonate platforms of the world, inasmuch as the sequence boundaries, as demonstrated in this study, correlate with the eustatic sea level curves.

Recognition of Sequence Boundaries in Pennsylvanian Outcrops of the Midcontinent: ABSTRACT

Abstract Subaerial exposure surfaces, and associated features are important and common components of shallow-marine Paleozoic sequences in the Midcontinent. Recognition and mapping of such surfaces are of economic importance because the distribution of reservoir facies is closely associated with subaerial exposure. The absence of strong evidence of subaerial exposure in these sequences (e.g., unambiguous petrographic and isotopic data) creates problems in stratigraphic correlation at both the reservoir and regional scales. However, exposure surfaces and associated paleo-water tables can be recognized by variations in the concentration of the three most commonly occurring radioactive elements (i.e., uranium, potassium and thorium). These variations in the concentration are readily detected in both the surface and subsurface using the spectral gamma-ray log. Detailed field description, petrographic examination, and geochemical analysis were used to understand the relationship between the concentration of radioactive elements, and the style of alteration associated with subaerial exposure at the overlying sequence boundary in selected Pennsylvanian carbonate sequences. In several sequences distinctive change in uranium concentration is observed at the contact of the vadose and phreatic zones. On the basis of detailed examination, the uranium anomalies cut across primary sedimentary structures, and are proposed to be the result of selective inclusion of uranium in micrite cements and associated clays concentrated at the contact between the vadose and phreatic environments. Development of criteria to recognize this distinctive signature in both the surface and subsurface may assist in correlation of individual sequences and will aid in understanding genetic mechanisms that control reservoir development in similar shallow-marine Midcontinent reservoirs.

Sedimentology and Paleogeography of the Natih Carbonate Platform in the Oman Mountains

Field study of the Natih Formation in the Jebel Akhdar and the Oman foothills allows us to establish a new stratigraphical and sedimentological model of this important hydrocarbon reservoir unit. Thanks to the study of rudists and the discovery of ammonites, a new precisions can be given to the chronostratigraphy of the Natih Formation. It was especially demonstrated by the presence of Hippuritids (rudists) that the top of the Natih Formation matches the Cenomanian/Turonian boundary and corresponds to either rudist-rich layers or hard grounds and condensed levels. The stratigraphical correlations between several outcrop section allow one to establish a conceptual sequence stratigraphic model which can be compared to the subsurface by using the Natih subdivisions [open quotes]A to G.[close quotes] The recognition of sequence boundaries, maximum flooding surfaces, and system tracts might help to understand the seismo-stratigraphic expression of the Natih interval in the subsurface. Furthermore, this sequence stratigraphic model clearly illustrates the interfingering of the carbonate reservoir intervals and the organic-rich units (Fitri Mb) at the top of the Natih Formation. We also have identified a clear zonation from deeper marine to shallow-marine carbonate deposits, the most significant of which are the rudistid facies. They form banks, thickets, andmore » biostroms and do not constitute anomalous build ups such as bioherms. Good leaching potentials generally are related to these rudistid facies, especially when they are very rich in skeletal aragonite from the Caprinids shells. Significant primary porosity may be related also to the Hippuritid skeletal cavities at the top of the Natih. Reservoir potentials can be enhanced if these Caprinid-rich intervals are related to exposure surfaces such as the top Natih E and the top Natih A.« less

Estuarine, barrier-island to strand-plain sequence and related ravinement surface developed during the last interglacial in the Paleo-Tokyo Bay, Japan

Recognition of sequence boundaries and transgressive surfaces (i.e. ravinement surfaces, RS) is now known to be of great importance in stratigraphy. The sedimentary features of deposits immediately above a transgressive surface are well exposed in the Upper Pleistocene Kioroshi Formation of the Kanto Plain in central Japan. The formation comprises mainly coastal and shallow-marine deposits (estuarine, barrier-island and the strand-plain systems) which accumulated along a wavedominated coast in the Late Pleistocene, i.e., the last interglacial to last glacial period. The Kioroshi Formation is bounded above and below by sequence boundaries that formed in the lowstand periods correlative to the glacial periods of oxygen isotope stages 4 and 6, respectively. A significant transgressive surface that was formed by landward migration of barrier islands during the transgressive interval, the ravinement surface (RS), is found within the deposits of the upper shelf environment. This ravinement surface is characterized by the exotic nature of the overlying sediment veneer (pebbles, shells and scattered mud clasts) which is poorly sorted. The RS shows a very flattened erosional surface in the shore-parallel sense, and the gradient of the surface in shore-normal sense is calculated as 0.0021, where the syndepositional tectonic movement is revised. The RS commonly cuts through the lower sequence boundary. However, in the places where the river or tidal channel valleys incised, the valley-filling sediment shows a deepening-upward sequence recognized as a transgressive systems tract and the RS can be clearly distinguished from the lower sequence boundary.

Sequence stratigraphic analysis of the Aptian-Albian Lower Greensand in southern England

Facies analysis and its use in sequence stratigraphy is described using the Lower Greensand of southern England as an example, and the resulting interpretations are outlined. This work forms an independent test of sequence stratigraphic techniques and the Exxon ‘cycle chart’. Sequence stratigraphy is a very powerful tool in the analysis of problematic successions, although some modification is made here in emphasizing the use of the transgressive surface. Changing palaeoenvironmental conditions, and the effect that palaeoceanography has on the recognition of sequence boundaries, are also discussed. Broad agreement is found between these results and the cycle chart, although changing tidal conditions alter the criteria available for analysis of the Aptian-Albian Lower Greensand succession. Comparison between this study and sequence stratigraphic analyses conducted in the more condensed type sections of England and France shows that further sequence boundaries are recognizable.

Outcrop Recognition of Sequence Boundaries in Carbonate Rocks: ABSTRACT

Outcrop Recognition of Sequence Boundaries in Carbonate Rocks: ABSTRACT