|Title:||The Application of Seismic Velocity Pseudo Section in Interpreting Subsurface Stratigraphy in Offshore Niger Delta, Moonstone Field|
|Author(s):||Eze Stanley, AyuaKuma Joshua, Nnorom .S. Lotanna, Ede Tiekuro|
|Publisher:||IOSR Journal of Applied Geology and Geophysics (IOSR-JAGG)|
|Keywords:||Seismic amplitude Velocity sag porosity discontinuity pseudosection|
Recordsoffour (4) seismiclinesfromMoonstone Oil-field Niger Delta have been interpreted forthe structures, anticlinalhorizons and faults present. Porosityand velocity Pseudo Sections were constructedand interpretedfor the observed structures. Velocity pseudo section was usedtoidentify hydrocarbon accumulationzoneby checking for sudden reduction (velocity break and discontinuities) in its value, and regionalcompactiontrends was deducedsince interval velocity value increases with depth. The Porosity Sections for seismic lines M-016, M-020, M029, and M-031 show changes in porosity variation with depth. For line M-016, porosity discontinuities is seen at 4000-5000m, for line M-020 porosity discontinuities is seen at 3000m and 5000m respectively. The Porosity Sections for seismic lines M-029 and M-031 show the changes in porosity with depth and discontinuities at 5500m and 4000m respectively. Horizons were picked where there are lithological Changes. Velocity pseudo section for LinesM-016, M-020, M-029,andM-031,delineated four (4) layers. Layer1with velocity rangingfrom1490m/sto 2248m/s, ischaracterized by relatively strong and continuous reflection amplitudes. This layer consist of loose and porous sands of the Benin formation. Layers2and3aresimilarintheirreflectionpatterns, but with strongeramplitude reflections,mostofwhicharediscontinuous.Thevelocity within these layers vary from 2317m/sto 3532m/s and this sectiondoes not show the faults characteristic of the Agbada formation since geologic features are concentrated downdip. However, a porosityrangefrom 20-35% was obtained within these layers and is consistentwiththeestablished porosity values within the Agbadaformation in the Niger Delta. The fourth layer occurringatadepthofabout3000mischaracterizedbyirregularand discontinuousreflections with velocitiesrangingfrom2547m/sto3200m/s.Thenatureofthe reflectionsischaracteristicofshales,indicating a change in lithology from sandy shalesatthebaseoflayer 3 into shales of layer 4, this layer is the Akata formation. Thevelocities of the layers increases withdepth which is asa result ofthe increasingcompaction, depth of burial and age. ThePseudo-sectionswerecomparedwiththeSeismicamplitude sectionsofthefourlines (M-016, M-020, M-029 and M-031) and theresults obtainedshowagoodmatchbetweenthesetsof sections. The seismicrecordshow resultsofa subsurfacegeology thathasthreemajor subdivisions whichis Inagreementwiththeestablishedsubsurfacegeology oftheNigerDelta. Theresultsobtained in this study areveryusefulinthesitingofappraisalanddevelopmentwells however, it should be carried out on the entire field.
Keywords: Seismic amplitude, Velocity sag, porosity discontinuity, pseudosection.