Study On Sedimentary Characteristics Of The Lower Part Of The Third Member Of Shahejie Formation In Luojia Area,Zhanhua Sag

As the development of the major oil and gas in the conventional reservoir hasbeen deeply maturated, the focus of the oil and gas exploration has turned to the shalereservoirs. The shale in Zhanhua Sag has great potential of hydrocarbon and hasbecome important exploration targets. Based on cores, well logs and other testing data,using the theory of sedimentology and sequence stratigraphy, the sedimentary featuresof the different shale types, the depositional environment and sequence stratigraphy ofshale in Lower part of the Third Member of Shahejie Formation, Luojia area, ZhanhuaSag are studied in this paper.Seven lithological types which contain laminated boundstone, laminated marl,laminated calcareous shale, massive mudstone, silty mudstone, massive marl andmassive calcareous mudstone are identified using the data of thin section, whole rockX-ray diffraction, scanning electron microscopy, field emission electron microscopyand other analyzing data of the two key wells Luo67and Luo69. The different typesof shale have represented varied sedimentary environment and has distinctivedepositional process. The shale sequence stratigraphic framework is always controlledby their depositional process. Based on this theory, climate, terrigenous clastic influxand relative lake level are taken account as three major influencing factors to controlthe formation of sequence stratigraphy, and indicators like organic matter content,mineral content, radioactive loggings and trace elements are selected to help dividethe sequence stratigraphy. The deep water sequence stratigraphy of study area in theLower part of the Third Member of Shahejie Formation is then built and consists oftwo third-order sequence and six systems tracts.A logging interpretation model of mineral content and a logging chart board toidentify seven major lithologies are proposed in this paper, based on the analysis ofthe cores and logging date from well Luo69. More than50wells in study area areintroduced by the above standard model and chart board to gain the mineral contentand lithological data, and therefore the contour map of mineral content andlithological facies distribution can be accurately mapped in plane among different systems tract. Then combined the analysis of lithological facies, pollen data,radioactive loggings, mineral assemblages and trace elements, the sedimentaryevolutional rule of Lower part of the Third Member of Shahejie Formation can berecovered.Four periods of sedimentary evolutions are proposed in this paper. Sequence1:(1) The lowstand systems tract (LST): in its early stage, the study area is in dryclimate and has few terrigenous clastics influx and thick limestone deposited, and thenin its later, water slowly rising stage the climate has changed into wet and warmresulting the prosperity of benthic algae and finally caused the unique deposits ofboundstone in certain areas;(2) The transgressive systems tract (TST): it has a morewarm and wet climate than LST with terrigenous clastics influx also increasingslightly. In this period phytoplankton algae have replaced the benthic algae to be thedominant algae;(3) The high systems tract (HST): the climate has become less warmand wet, and the large amount influx of terrigenous clastic has made the increase ofsiltstone and bioclastic deposits.(4) Sequence2: it has presented a more warm-wetclimate, general stronger terrigenous clastic influx and deeper lake level thanSequence1. The depositional characteristics study of shale in study area is of greatimportance to guide the future hydrocarbon exploration.