ORGANIC GEOCHEMICAL CHARACTERISTICS OF THE LOWER PALAEOZOIC CARBONATE ROCKS IN THE HUANGHUA DEPRESSION, THE BOHAI BAY BASIN

A study has been conducted on the organic geochemical characteristics of the Lower Palaeozoic carbonate rock samples from the Huanghua Depression,the Bohai Bay Basin.The results show that the distribution of n-alkanes in chloroform bitumen A is characterized by a single peak type,with the main peak carbon at nC₁₆ or nC₁₈;the OEP is approximately equal to 1 and the ratio of C₃₁22S/(22R+22S) homohopanes is between 0.58～0.63,indicating that the organic matter has evolved into a high maturity stage.The Pr/Ph ratio is commonly close to or slightly greater than 1.0,reflecting a depositional environment under a week reducing or week oxidizing condition.In contrast,the distribution of n-alkanes in chloroform bitumen C is characterized by a double peak type,suggesting that the maturity of the organic matter in chloroform bitumen C is lower than that in chloroform bitumen A.The Pr/Ph ratio approaches 1.0,similar to that of chloroform bitumen A,also reflecting a week reducing or week oxidizing depositional environment.The results also show that the carbon isotope pattern of the individual n-alkanes in chloroform bitumen C is similar to that in chloroform bitumen A in most samples,indicating that the carbon isotope of the individual n-alkanes in chloroform bitumen C had not been significantly affected by the post-diagenetic geological processes,and the main characteristics of the organic matter in the early stage has been kept.Therefore,it can be used in paleoenvironmental restoration,and can provide the basis for identifying the potential Lower Palaeozoic primary oil-/gas-reservoirs in the Bohai Bay area.