Abnormal Formation Pressure And Its Relationship With Hydrocarbon Accumulation In The Xihu Sag

The Xihu Sag lies in the east of the East China Sea Shelf Basin. The abnormal formation pressure is very common in the lower section of the Oligocene Huagang Formation and the Eocene Pinghu Formation, but little is known about the relationship between the abnormal formation pressure and the hydrocarbon accumulation. This paper discusses log-based individual-well formation pressure interpretation with balancing depth method, Eaton method and Bowers method, which is calibrated with measured formation pressure and then combined with lithology and maturity data of organic matter to establish individual-well pressure profile for the Xihu Sag. The paper also deals with lateral pressure variation based on individual-well pressure data and high pressure genesis based on the relationship between sonic velocity and vertical effective stress and other information, e.g. the rate of sedimentation, lithology, organic matter maturity, etc. As per fission track tests for apatite, inclusion data analysis and numerical simulation of overpressure history in the region of interest, this paper discusses the relationship between formation pressure and hydrocarbon accumulation and presents pressure-dominated accumulating modes for the Xihu Sag and potential zones of enrichment. This paper presents the following conclusions.(1) Individual-well and connected-well pressure profiles were built with balancing depth method, Bowers method and Eaton method. The results show that the first method is applicable to shallow pressure estimation and the latter two methods are applicable to middle and deep depth pressure estimation. For the study area, Eaton exponent should range between 1.3 and 1.6.(2) Through the calculation of seismic interval velocity, multiple seismic pressure profiles has been built, and the estimating result are almost consistent with the measured or calculated logging data, which proved the correctness and reliability of the method of seismic interval velocity, according to this, the overpressure could be predict in not yet drilled areas or wells without the restriction of drilling data.(3) Based on the predicted overpressure results from logging and seismic data, the top interface of the abnormal pressure in the western slope belt and the pinghu belt is at the second member of the Pinghu Formation, and the abnormal pressure is developed in the third and fourth members of the Eocene Pinghu Formation. However, in the west sub-sag area, the abnormal pressure is detected at the lower Oligocene Huagang Formation, in the central inverted structural belt, the condition is the same. As lateral and plane pressure variation, pressure coefficient would change rapidly in the slope zone from the sag to the salient, which indicates the driving force of hydrocarbon migration and accumulation. Abnormal pressure is common in the west sub-sag. For central inverted structural belt close to the east and west source sub-sags, some wells were detected with abnormal pressure in the lower Huagang Formation, however it is anticipated that abnormal pressure commonly exists in the underlying Pinghu Formation.(4) In view of the genetic mechanisms, the overpressure in the study area is the product of one or two of following factors, e.g. undercompaction, hydrocarbon generation and fluid conduction. For the Pinghu Formation, abnormal pressure in mudstone sections in the second and third members is caused by undercompaction plus hydrocarbon generation and that in the fourth member is caused by hydrocarbon generation plus undercompaction. In the deep layers of the west sub-sag and central inverted structural belt, abnormal pressure originates from hydrocarbon generation plus undercompaction and the overpressure is enhanced by hydrocarbon generation. The overpressure in the reservoir is mainly caused by fluid conduction, which includes hydrocarbon charging from high-pressure source rocks into reservoirs and water dehydration from undercompacted mudstone into neighboring reservoirs.(5) As T-t tracks from apatite fission simulation, Pinghu slope zone mainly subsided since 32 Ma while only uplifted occasionally to experience denudation. Tectonic uplifting and denudation in the west sub-sag(around Well XCA-2) mainly occurred from 8.0~7.7 Ma to 5.5~4.5 Ma with the thickness denuded of about 120 m. Central inverted structural belt(around Well ZHS-18) experienced uplifting and denudation in the similar period with the west sub-sag, i.e. the Longjing movement, with the thickness denuded of 270~290 m. Rapid burial and geotemperature increase occurred after 5.5~4.5 Ma, which has created ideal conditions for the overpressure caused by hydrocarbon generation of conduction the basis of and it is also the primary period of hydrocarbon generation and accumulation.(6) The inverted pressure history and inclusion data analysis show that there is a good correlation between overpressure and maturity. The starting time of abnormal pressure surface in each structural belt is consistent with the satge time of hydrocarbon charging and the depth of abnormal pressure is also consistent with the variations of formation temperature, geothermal gradient and the threshold of hydrocarbon generation. High formation temperature would facilitate hydrocarbon maturation, which would facilitate the development of overpressure.(7) Based on the summary of thin section data, the Pinghu Formation is superior to the Huagang Formation in primary porosity and the latter has higher secondary surface porosity than the former. The preservation of primary porosity in deep Pinghu formations is related to undercompaction and pressure boost resulting from early rapid subsidence, which may relieve mechanical compaction from overlying formations. In accordance with the profiles of pore fluid pressure-reservoir property-buried depth in different structural belts, reservoir properties would depart from the compaction trend with increased buried depth and overpressure, which also means the reservoir property would be improved due to overpressure.(8) With the analysis of the relationship between sealing performance of overpressure and hydrocarbon reservoir distribution, it can be concluded that there may be 4 types of pressure-dominated accumulations in middle and deep zones, i.e. double-confined high-pressure dominated accumulation, double-confined accumulation in the pressure transition zone, semi-confined accumulation in the pressure transition zone, and semi-confined normal-pressure dominated accumulation. The second and fourth types mainly occur in the Pinghu structural belt in the west slope zone. In detail, the former two types occur in Zone Ningbo 13 in the north Pinghu structural belt; the latter two types occur in Zone Ningbo19; the second and fourth types occur in Zone Ningbo 25; all these four types occur in the central Pinghu structural belt. The first type, followed by the second type, occurs in the west sub-sag and central inverted structural belt.(9) In accordance with hydrocarbon genetic mechanism based on the classification of pressure-dominated accumulations, the pressure-dominated accumulation mode of Xihu Sag has been constructed, which show that those regions with double-confined transitional pressure and semi-confined normal pressure would be favorable for hydrocarbon accumulation. Reservoir beds in these regions would be sealed by both overlying caprock and high pressure. Hydrocarbon generated by internal high-pressure source rocks and source kitchens in neighboring deep sags would be expelled, due to driving force of high pressure, into salient and slope area which has low altitude.