A week of expected banner profit reports from the nation's major petroleum companies began yesterday, as Standard Oil Co. of Indiana posted a 40 percent increase in total profits during last year, compared with 1978.

But Standard, which markets products under the Amoco name, emphasized the U.S. spending to find and develop oil or natural gas reserves rose 57 percent over the same period.

The profit reports this week come at a crucial time, as House and Senate conferees are nearing final agreement on a $227 billion "windfall" oil profits tax. Major oil firms, including Standard, are seeking to emphasize the amount of their investments in future energy sources to help put in perspective the record profits they are reaping -- primarily the result of soaring world crude oil prices.

For example, Exxon Corp., by far the largest petroleum company, placed advertsiements in 328 newspapers yesterday at a cost of about $500,000, reporting that the company plans record energy investments of $6.6 billion in 1980 and noting that worldwide energy investments for the last decade totaled 1 1/2 times as much as its profits.

Standard of Indiana, the nation's sixth-largest oil company, invested a record $3 billion last year for worldwide projects and exploration, an increase of 35 percent from 1978 that was made possible by the higher earnings, according to Chairman John E. Swearingen.

For the year, Standard earned $1.5 billion ($10.23 a share) compared with $1.08 billion ($7.36) in 1978. Revenues increased 24 percent to $20 billion and the amount of spending for development of crude oil and natural gas reserves in the U.S. alone rose 57 percent to more than $1.5 billion -- topping the level of profits for the year.

In the fourth quarter alone, Standard profits jumped 70 percent to $368.7 million ($2.45 a share) from $217 million ($1.48), as revenues rose 41 percent to $6.1 billion.

As strong as Standard's gains were in 1979 -- and Wall Street analysts are forecasting that the other major companies will post similar, glowing results -- the Chicago-based company actually would have earned far more profits if not for adoption of an accounting change at the end of 1979.

Full-year and fourth-quarter profits, as reported yesterday, were reduced by a steep $165 million ($1.12 a share) and $122 million (81 cents), respectively, because of the accounting change.

A company statement described the change as related to crude oil inventories -- extending the last-in, first-out (LIFO) accounting method to certain crude oil shipments in transit that previously were valued on a first-in, first-out (FIFO) basis.

This change, which a number of companies with large inventories have adopted to reflect the impact of inflation, matches current costs with current revenues and is widely considered a more realistic assessment, given soaring world crude prices.

Worldwide crude oil and natural gas production was off 18 percent during the year to an average of 849,000 barrels day, because of loss of production in Iran.

In the U.S. at the same time, Standard's petroleum earnings declined to $650.8 million from $713 million.