The Study On Sequence Stratigraphy, Sedimentary Systems And Favorable Reservoir Prospection Of Cretaceous In Termit Depression, Niger

Termit depression is a part of East Niger Basin belonging to the Central andWest African Rift System which consists of a series of genetic-related Meso-Cenozoicbasins. Recent exploration results indicate that the Cretaceous formation of Termitdepression in Niger has good oil and gas potentials. But the work area is still in lowdegree of exploration and in short of enough drilling wells that reveal Cretaceousstrata, seismic data with good quality and the experimental analysis materials. Thesegreatly restrict the prediction on distribution of favorable sedimentary facies,reservoir and the further hydrocarbon exploration.This research aims to carry on comprehensive study of sequence stratigraphy ofCretaceous in Termit depression of Niger, guided by the theories of sequencestratigraphy and sedimentology. The paper has established the sequence frameworkof Cretaceous formation, then studied the types of sedimentary facies anddistribution of sedimentary system and finally predicted distribution of favorablereservoir. This research is of crucial significance to thorough understanding ofevolution of depositional system and distribution of favorable reservoir and furtherhydrocarbon exploration of Cretaceous strata both in theory and practice.Based on the study of mud logging, well logging and seismic data, threesecond-order and seven third-order sequence interfaces of Cretaceous in Termitdepression are identified according to the methods of parasequence sets stackingpatterns,△LogR, seismic reflection terminal relations. The Cretaceous thus could bedivided into two second-order sequences and nine third-order sequences(KSQ1~KSQ9).This paper has identified the types and characters of sedimentary facies ofCretaceous in Termit depression on the basis of paleontology and elementgeochemistry data which reflect the general sedimentary environment of Cretaceousand the combined methods of well logging facies and seismic facies. Then, the paperhas studied the plane distribution of the sedimentary systems in each sequence ofCretaceous by analyzing single well and well-tie facies, seismic facies and seismic attributes. The results show that Sequence KSQ1 and KSQ2 of Lower Cretaceousmainly developed braided river delta and delta—lake sedimentary system, and thebraided river delta mainly distributed in the west steep slop zone while the delta-lakesediments distributed in the east gentle slop zone. Sequence KSQ3 is composed ofdelta—lake deposits originated from both west and east sides and the long axisdirection of the depression. During the period of Sequences KSQ4~KSQ7, marinetransgression scale became larger and the sea water gradually retreated in the periodof Sequence KSQ8. Sequences KSQ4~KSQ8 mainly developed onshore depositsand delta-marine deposits from the east of the study area to the west. SequenceKSQ9 is characterized by braided river delta sediments.According to the study of petrology, diagenesis, reservoir space types and reservoirphysical properties, this paper describes and classifies favorable reservoir ofCretaceous in details. It is considered that the delta front sandsones, braided riverdelta front sandsones and tempestites of inner shelf are favorable reservoir types ofCretaceous in Termit depression. Based on the synthetical analysis of sedimentaryfacies and reservoir evaluation, the paper predicts the plane distribution of favorablereservoirs in each sequence of Cretaceous and concludes that the delta sand body inSequence KSQ8 is the most favorable exploration target of Cretaceous in Termitdepression.