Practice of hydrocarbon exploration in recent years indicates that Tarim Basin has following geological conditions for formating huge oil-gas fields:1. The dovelopment of large hydrocarbon-generating depressions

especially the Manjiaer Cambrian-Ordoviciandepression with a total area of 80000km

2

source rocks of 2000m thick and total hydrocarbon resources of 6000 million ton. 2. Uplifts of long-period developmen between the large hydrocarbon-generating depressions

especially the Shayar and Central Uplifts

which were the direction of hydrocarbon migration

and were favourable for hydrocarbon accumulation. 3. Low geothermal gradient

the mean geothermal gradient of Mesozoic and Cenozoic was about 2℃/100m

and that of Paleozoic was 2.6-2.7℃/100m. This was fairly favourable to hydrocarbon generation in late stage for major source rocks (Cambrian to Ordovician) in the basin

and slowed down the diagenetic evolution of Silurian to Tertiary clastic rocks

Therefore

the development zone of secondary pores is not only wide in longitudinal distribution (generally greater than 2000m)

but also very deep (for example

the porosity of the clastic rocks which is buried in the depth of 4300-5800m is still up to 15-25%). 4. Good preservation conditions in the later stage due to weaktectonic movements in the Meso-Cenozoic

and the development of regional cap beds which are characterized by extensive distribution

large thickness and good sealing property. 5. Well developed faults and unconformity surfaces as the main passage ways for hydrocarbon migration and accumulation

and which obviously improved reservoir property of Lower Paleozoic carbonate rocks

especially the unconformity surfaces of the PreMesozoic that usually formed weathering surface reservoirs.