Hydrocarbon accumulation conditions and key exploration and development technologies for PL 19-3 oilfield

The PL 19e3 Oilfield is the only super-large monolithic oilfield with oil and gas reserves up to 1×10^(9) t in the Bohai Bay Basin,and it has been successfully developed.Exploration and development practices have provided abundant data for analyzing formation conditions of this super-large oilfield.On the basis of the exploration and development history,fundamental reservoir features,and with available geological,geophysical and test data,the hydrocarbon accumulation conditions and key exploration&development technologies of the PL 19e3 Oilfield were discussed.The key conditions for forming the super-large Neogene oilfield include four aspects.Firstly,the oilfield is located at the high position of the uplift that contacts the brachy-axis of the multi-ridge slope in the biggest hydrocarbon-rich sag in the Bohai Bay Basin,thus it has sufficient hydrocarbon source and extremely superior hydrocarbon migration condition.Secondly,the large-scale torsional anticlines which formed in the Neogene under the control of the Tanlu strike-slipping movement provide sufficient storage spaces for oil and gas preservation.Thirdly,the“multiple sets of composite reservoir-caprock assemblages”developing in the special shallow-water delta further contributes greatly to the effective storage space for oil and gas preservation.Fourthly,due to the coupling of the uplift and strike slip in the neotectonic period,extensive faulting activities constantly released the pressure while the late period massive hydrocarbon expulsion of the Bozhong took place at the same time,which assures the constant and intense charging of oil and gas.The super-large PL 19e3 Oilfield was controlled by the coupling effects of all those special geologic factors.In view of this oilfield's features(e.g.violently reformation caused by strike slip,and the special sedimentary environment of shallow-water delta),some key practical technologies for exploration and development have been developed.Such technologies include:the special prestack depth migration processing for gas cloud zones,the prediction of thin interbed reservoirs based on high-precision inversion of geologic model,the reservoir description for the shallow-water braided river delta,the quantitative description for remaining oil in the commingled oil reservoirs with wide well spacing and long well interval,and the well pattern adjustment for formations during high water cut period in the complex fluvial-facies oilfields.

Petroleum geology and sub-source hydrocarbon accumulation of Permian reservoirs in Jinan Sag,eastern Junggar Basin,NW China

According to the latest drilling and the analysis of the burial history,source rock evolution history and hydrocarbon accumulation history,the sub-source hydrocarbon accumulation characteristics of the Permian reservoirs in the Jinan Sag,eastern Junggar Basin,are clarified,and the hydrocarbon accumulation model of these reservoirs is established.The results are obtained in four aspects.First,the main body of the thick salified lake basin source rocks in the Lucaogou Formation has reached the mature stage with abundant resource base.Large-scale reservoirs are developed in the Jingjingzigou,Wutonggou and Lucaogou formations.Vertically,there are multiple sets of good regional seals,the source-reservoir-caprock assemblage is good,and there are three reservoir-forming assemblages:sub-source,intra-source and above-source.Second,dissolution,hydrocarbon charging and pore-preserving effect,and presence of chlorite film effectively increase the sub-source pore space.Oil charging is earlier than the time when the reservoir becomes densified,which improves the efficiency of hydrocarbon accumulation.Third,buoyancy and source-reservoir pressure difference together constitute the driving force of oil charging,and the micro-faults within the formation give the advantage of"source-reservoir lateral docking"under the source rock.Microfractures can be critical channels for efficient seepage and continuous charging of oil in different periods.Fourth,the Jingjingzigou Formation experienced three periods of oil accumulation in the Middle-Late Permian,Middle-Late Jurassic and Late Neogene,with the characteristics of long-distance migration and accumulation in early stage,mixed charging and accumulation in middle stage and short-distance migration and high-position accumulation in late stage.The discovery and theoretical understanding of the Permian reservoirs in the Jinan Sag reveal that the thrust belt has good conditions for forming large reservoirs,and it is promising for exploration.The study results are of guidance and reference significance for oil and gas exploration in the Jinan Sag and other geologically similar areas.

The Formation Conditions and Distribution Regularities of Lithologic Reservoirs in the Songliao Basin

The geological background of the fluvial and lacustrine depository system in the large mid-shallow layered depressed lake in the Songliao Basin has been proven to have the geological conditions for the formation of lithologic reservoirs over a large area. Because the rise and fall of the base level caused considerable differences in the space at different periods, the types of sand bodies varied in different geologic periods; but at the same time, because of the differences in burial depths and the relative differences in positions of the siltstones and source rocks, the formation conditions and distributive regularities for oil-bearing groups of subtle reservoirs also vary accordingly. In view of the reality of the thinly-interbedded lithologic reservoir in the deltaic frontal facies belt, we have conducted a series of research to study the thinly-interbedded subtle reservoirs with the help of high resolution sequence stratigraphy, pattern recognition and the geophysical recognition and prediction techniques for thinly-interbedded subtle reservoirs, thus achieving a better effect and optimizing several exploration target areas, each of which has an oil reserve of about 100 million tons. This has shed light on further exploration of new subtle reservoirs.

Hydrocarbon accumulation and key exploration & development technologies of Changning-Weiyuan marine shale gas field, southern Sichuan

China is rich in shale gas resources and has broad prospects for development. However, the quality ofshale gas resources in China is different from that in the United States, so independent unique technologies are urgently required in China. In this regard, PetroChina has carried out theoretical andtechnical researches systematically on shale gas in Longmaxi Formation in southern Sichuan according toits geology-engineering characteristics. Guided by the “three-control” theory for marine shale gasenrichment and high yield (i.e. sedimentary diagenesis controls reservoir, preservation conditions control accumulation, and continuous thickness of Type I reservoir controls production), six key explorationand development technologies in southern China have been innovatively developed for marine shale gaswhich experienced multi-stage tectonic evolution. The technologies include comprehensive geologicalevaluation technology, development optimization technology, optimal and fast drilling technology forhorizontal well, volume fracturing technology for horizontal well, factory-like operation technology, andefficient clean production technology. These technologies have enabled the large-scale commercial recovery of shale gas. By the end of 2019, PetroChina had obtained proved geological reserves of1.06
1012 m3, had built production capacity of 10
109 m3, and had cumulative gas production of20
109 m3 in southern Sichuan. The remarkable results of application are of great significance forvigorously developing shale gas industry to reduce external dependence, ensure national energy security,and realize the strategy of "ecological priority and green development".