Keywords: 
• sequence stratigraphic framework /
• sequence boundary /
• development model /
• Cretaceous /
• Oriente Basin

Abstract: The Oriente basin of Ecuador has produced a substantial amount of oil over the past years. The Cretaceous is the main exploration target in the northeastern block of Oriente Basin in Ecuador. But now in the study area, there are certain differences on the understanding of the sequence development number, the position of the sequence boundary and the sequence composition among different scholars. The classification projects of sequence stratigraphy haven't been systematically recognized. It greatly restricts any further exploration of hydrocarbons. Therefore, it has great theoretic and practical significance to carry out thorough researches on sequence stratigraphy of Cretaceous in northeastern block. Based on the comprehensive analysis of core, well logging and seismic data, sequence boundaries of the Cretaceous in northeastern block of Oriente Basin are identified according to seismic reflection terminal, the stacking patterns of sequence as well as changes of lithology and isochronous stratigraphic framework of the Cretaceous including Hollin and Napo Formation is established. Hollin and Napo Formations are divided into one second order sequence (KSS1), six third order sequences (KSQ1-KSQ6), sixteen fourth order sequences and thirty-four fifth order sequences. The KSQ1-KSQ5 has complete system tract composition which develops SMST, TST and HST system tract. The KSQ6 only develops SMST system tract. According to synthetic records for calibration of the seismic profile and combining with drilling and logging sequence stratigraphic division, seismic as well as drilling and logging data consistency of sequence stratigraphic division of Cretaceous in the study area has been implemented.
In the sequence division of Cretaceous in the northeastern block of Oriente Basin, the characteristics of sequence boundary recognition at all levels are obvious. Second order sequence boundaries correspond to typical seismic profile unconformity truncation feature. Third order sequence boundaries include partly onlap, truncation and downlap seismic reflection termination. Drilling profile are all lithological abrupt change surface, showed the abrupt change surface in the well logging due to the sediments from the shallow water to the deep water. The lowstand of fourth order sequence are shelf marginal systems tracts. It is difficult to identify fourth order sequence boundaries in seismic line, but it is easy to identify in well logging. The key element to identify interfaces is to determine the first flooding surface and the maximum flooding surface. Mainly based on the change of parasequence group staked style in well logging, showed the conversion interface of aggradation-retrogradation-progradation. Fifth order sequence boundaries correspond to the main flooding surface. The identification of interfaces are depending on the thickness of the formation development, the overlay style, the well logging response characteristics and the basic upward shoaling succession cycle of the relative sea level.
Typical mixed clastic and carbonate gentle dipping slope sequence stratigraphy development mode has been identified in the study area. Cretaceous as a relatively complete second order sequence, experienced a complete sedimentary cycle that the relatively down-up-down sea level. Also, the third order sequence has apparent transgression-regression cycle which responds to sedimentary environment that undergo multi-period transitions from tidal flat to restricted platform, to inner shelf subaqueous shoal and to outer shelf mudstone. Then the sea level drops, meanwhile the sedimentary environment evolves into inner shelf limestone bank and restricted platform. Establishment of isochronous stratigraphic framework of the Cretaceous will provide scientific basis for predicting the distribution of favorable sand body in the northeast block, Oriente Basin.