The investigation of the offshore natural gas fields in the Gulf of Mexico ordered by Interior Secretary Cecil D. Andrus could take as long as eight months and involve as many as 1,000 people in and outside the department.

That's the appraisal by sources close to a preliminary investigation of four big gas fields that revealed unexplained drops in production and non-producing reserves of 767 billion cubic feet in the four fields.

The investigation ordered by Andrus will involve 202 fields in the Gulf of Mexico, which last year produced almost 3.5 trillion cubic feet of natural gas for the interstate market serving most of the eastern two thirds of the nation.

"Individual people will have to be sent out to study individual fields," said one source who asked not to be identified. "That means a lot of people and a lot of time, since there are more than 200 fields to be studies."

When he announced the investigation, Andrus said he hoped it would be finished in 60 to 90 days so that the crippling shortages of this winter would be fresh in mind when the findings are announced.

It now appears there is no hope for a quick study unless interior scales down the size. Sources said the investigation will take at least six months and probably eight.

A major question to be answered in the probe is whether any fields that could have produced gas were held back from production until after July 1, 1976, when the Federal Power Commission allowed interstate prices to rise from 52 cents a thousand cubic feet to $1.42. The FPC regulates the price of interstate gas, which supplies about 65 per cent of national demand.

Critics have long charged that producers are holding back in the hope that gas produced for the interstate market will be deregulated. The only gas not under FPC control today is intrastate gas, produced and consumed in the same state, such as Texas or Louisiana.

Another key question the investigation hopes to answer is whether the non-producing gas fields could have been tapped for production by drilling one or two new wells into each non-producing reservoir.

Many gas prodcuers have said that some fields are not producing because they do not have the money to sink the required number of new wells. The cost of sinking a new well runs as high as $1.2 million, depending on the depth of the water where the well must be drilled and the depth of the reserve the well must tap.

The extent of the non-producing reserves of gas in the 198 fields still to be investigated is another important question. The four fields investigated so far hold more than 700 billion cubic feet of gas that is not being produced.

Another question is why production in Gulf of Mexico gas fields is falling so rapidly. Three of the four fields looked at so far showed production declines in excess of 15 per cent a year and one showed a 26 per cent decline, whereas the average Gulf of Mexico gas field is understood to decline 12 per cent a year.